Simple answer: Performance, and not more RAM. ; - )
Although some 16-bit options expanded RAM, the AE 65C816 16 Bit Card (which was not released commercially) connected to an optional RAM card, one already used by existing applications.
The main practical application would (eventually) have been running a more powerful development environment - e.g. Merlin 16 - to write software for the 8-bit Apple II series.
I used Merlin 16+ on a IIgs for this purpose, but booting up GS/OS just to start Merlin was a pain. If you want to write IIgs software, you need a IIgs. But this card would've been fine for 8-bit software.
Even before the IIgs was released there were a number of options to upgrade existing Apple II computers with the 16-bit chips (65802 or 65816), in which case the only common ground would have been the 16-bit instructions - not more RAM - because only the products supporting the 24-bit addressing would provide that. It's obvious in retrospect that these fragmented products would remain a niche for development tools and tinkering, and that the IIgs would soon eclipse them.
For example the August 1986 issue of Open-Apple lists these 16-bit options, two with RAM:
1. 65802 chip. No RAM benefits. $50.
2. Com Log Apple16 65816 Co-Processor Board. Onboard 256K RAM. $395.
3. Checkmate Technology MultiRAM EX 65816 Co-Processor Card. Optional RAM card. $189.
4. Applied Engineering 65C816 16 Bit Card. Optional RAM card. $159. (Not released.)
5. Checkmate Technology 65C816 upgrade for CX Board for //c. Requires CX Board. $119.95.
6. Applied Engineering Transwarp Card l6 bit option (65802). Requires Transwarp Card. $89.
Open-Apple also states that only three 16-bit capable assemblers were available at that time:
1. Merlin Pro
3. S.C Macro Assembler
These were all 8-bit applications that could assemble 16-bit code, but Merlin 16 (which required a 16-bit CPU) was already in beta when the above was published, so there was already a market for 16-bit //e or //c software. When it was released there is evidence that development productivity benefited from the 16-bit chips, and Roger Wagner Publishing marketed 16-bit upgrade options without extra RAM to developers.
Apple IIe and IIc users can take advantage of [twice
as fast] Merlin 16 assemblies, a more powerful Full Screen Editor and
a newer, [five times faster] and more versatile Linker
just by swapping their existing 6502 or 65C02 microprocessor
with a replacement 65802 chip, available also from Roger Wagner
By special arrangement with Applied Engineering, Inc., Roger Wagner
Publishing is offering the powerful and popular Transwarp Accelerator
Card for the Apple IIe with a 65802 microprocessor already installed.
Using this card, Apple IIe owners can run Merlin 16 and assemble
programs faster than the Ilgs, in fact, four times faster than a
standard IIe running Merlin 8