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The original IBM PC had a slot for the 8087 floating-point coprocessor. This was a somewhat esoteric feature at the time – previous microcomputers had done all their floating-point in software – but it was sometimes used, and quite a few programs supported it; there is some discussion here: http://www.vcfed.org/forum/archive/index.php/t-38292.html

Notably:

  • Unlike nowadays, games never used floating point; even 3D calculations were always done as fixed point approximations.
  • AutoCAD, as expected, could use the FPU.
  • Ditto mathematical analysis and statistical programs.
  • Somewhat unexpectedly, so could some databases.
  • So could spreadsheets like Lotus 1-2-3. This is a tricky one. Spreadsheets should really use decimal arithmetic, because they are commonly used for financial calculations. VisiCalc correctly did this. Lotus 1-2-3 chose to use binary floating point. One could criticize this decision, but having made it, then it clearly made sense to support the FPU.
  • And of course scientists compiling their own number crunching code in Fortran would benefit from the FPU.

What was the reasons for people to buy the 8087? Given that spreadsheets were considered the killer application of personal computers, I would guess that to be the answer, but I would be interested if it turns out to be something else.

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    From what I recall, spreadsheet users were usually thirsty for more RAM to handle bigger files. But CAD users were thirsty for faster screen refresh, which led them to high-end graphics cards and FPU's. On the Amiga, FPU's were for rendering 3D graphics. Scientists convinced their patrons to buy them Unix workstations, which generally included FPU. – Brian H Nov 9 '20 at 18:20
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    Not the original 8087, but in 1994 I was working at a geothermal consultantcy which had a range of PCs - older 386s, 486s and as a new developer I got a new shiny Pentium P75. One of the junior engineers got a co-processor for their DX-33 386 second hand just to speed up their CAD package (not AutoCad but can't recall what it was). I did some benchmarks as we had an in-house Wellbore simulator using Paradox and Turbo Pascal + Fortran DLLs. Sped up our apps a lot, but 486 and Pentiums were replacing 386s, so only 387 I ever used – GrantB Nov 9 '20 at 18:24
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    I couldn't say which was the most common reason. The coprocessor was expensive rather than esoteric: I seem to recall paying around US$ 500 or more for an 8 MHz 8087 I bought in 1986, which I used for scientific and engineering calculations. My very first coprocessor-based computation was of the Mandelbrot set, however :-) – njuffa Nov 9 '20 at 18:40
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    A friend of mine had the 8087 and used it for CAD. – snips-n-snails Nov 9 '20 at 19:00
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    Re. FPU use in spreadsheets, the 8087 has a BCD type, so it could perhaps be used for financial calculations too without necessarily running into binary floating-point problems. (I’m not sure offhand what the conversion rules were, so there might still have been issues.) – Stephen Kitt Nov 9 '20 at 19:52
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Without detailed sales and usage figures, “the most common” is probably impossible to answer, and even considering only the 8087, the answer depends on “when” — reasons to buy 8087s changed as programs using it became available.

The more open plural variant of your question is largely answered in the question.

Byte’s 1984 “Guide to the IBM PCs” gives a good overview of the state of the PC market at that point in time, and spends quite a lot of space discussing the merits of the 8087. Its adverts provide some insight into what would have been possible at the time; the MicroWay ad on page 116 in particular. It mostly lists development packages supporting the 8087, but also a few “end-user” software packages: various graphics packages, an intriguing “Optical Design Program” (cheap at $3,000), and SuperCalc III. SPSS also features in its own ad, and a few other statistical packages appear elsewhere. The 8087 performance article mentions, regarding spreadsheets, that “their 8087 implementation really pays off in iterative financial analysis”.

So as you surmise, spreadsheets could have accounted for a large number of 8087 sales (whether for financial analysis or graphs), but arguably statistics programs could have as well, and in fact, given the state of the market and typical uses of PCs at the time, bespoke programs used for a variety of purposes.

The ads and magazine coverage also show the cost of the 8087 at the time — $230 as an official IBM option for its computers, but available for typically $175 on its own or $150 as part of a number of packages (with 8087-using software).

The same magazine shows another use of the 8087, which doesn’t quite fit into your question, but would have likely contributed to quite a few sales (albeit not as an add-on coprocessor): the IBM XT/370, which featured an apparently fully-IEEE-compliant variant of the 8087 used to implement the 370 mainframe’s floating-point operations.

Well-known programs which required FPUs appeared later on, as far as I can determine; examples include later releases of AutoCAD, and significantly, 3D Studio (see these interviews of Tom Hudson and Gary Yost if you’re interested in the latter’s history, and John Walker’s The Autodesk File). For such programs, the cost of an FPU was small compared to the cost of the package (3D Studio was heralded as excellent value for money when it released at $2,999, although if I remember correctly that quickly went up to $3,495). 3D Studio was perhaps the largest contributor to Weitek sales...

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