Just what was it used for?
The answer is in the name - to display graphics
It doesn't make much sense to produce a list of application, but lets look at the core issue of having a screen at all: There were no of-the-shelf graphic terminals - or (CRT-based) terminals - at all. A terminal was a card punch, a tape punch (and their counterparts) or a typing terminal like a FlexoWriter, IBM Model B or C, or any other TTY (*1).
Before vector displays, every output has to be done using either any of the above (hole or text based) devices or a plotter. At that time plotter weren't a comodity perhiperal, but devices with special custom interfaces, usually analogue. They were quite expensive, easy calling 40,000 USD and above. Using them was still a (comparable) slow process taking anywhere from 10 minutes to several hours. Getting to see a result within seconds on a screen was a game changer.
The fact that "most" (*2) were delivered with a Type 30 is simply due the the display being the most singular USP for the PDP-1.
At that time (1959), competition was the IBM 740/780 setup which cost 40,000 USD/year in rent. Just for the Display. A computer had to be rented as well. And even 5 years later, in 1964 the IBM 2250 Graphics Display Unit Started at 280,000 USD (sales price) for a basic configuration (*3).
A 120+ grand PDP-1 with display was already a bargain if only used as terminal to some existing comuter. And with the computer thrown in "for free", it enabled small (*4) departments getting both a once.
*1 - See this answer for a partial overview of terminal development.
*2 - At that point it might be important to keep in mind the the phrase "Most systems" sounds bigger than it is, as in total less then 60 PDP-1 were ever build.
*3 - Though, being expandable to 4 displays, this may put a per workstation price at or even below 100,000 USD :)
*4 - Well, relative compared to the ones that could afford an IBM 704 with 740/780. It'll be still an investment worth more than a million USD of today's money.