Some professional CD-i players offer a floppy drive so you can use a 3,5" disc to transfer data or to update drivers. These professional players were mainly used for either business purposes or in classrooms for educational purposes. The floppy drive was there for additional storage capabilities. It was used for driver updates but also for saving data that was created in professional applications like in classrooms: scores, values, to share it between players and in some cases between CD-i and MS-DOS.
The floppy drive in the professional line of CD-i players (those that were not for development like this CD-i 615) was used to save data from quizes and trainings, to swap between players, purely for business and educational areas. Stats, databases, small updates, those kind of things. I think in principle CD-i developers could have programmed CD-i games in such a way that the game data could be saved on a floppy (but that's speculation) but it's a simple fact consumer player were not equipped with floppy drives :) -
cdifan used these floppy drives in the past during his time as a CD-i developer: "I think the floppy drives where used mostly for verification tests and quick program tests. You could also use them for getting screenshots etc. off the CD-i (provided your program could generate them, of course). I remember often using the floppy for preloading a RAM disk (on the 605 you can make these so they survive a reset) with basic OS9 system utilities like dir, copy, list...""we used them for tests. But basically, those were OS9 system floppies, it looked and felt a bit like MSDOS. So you could do i.e. file operations on a command line, just like DOS", according to a former CD-i tester..
Finally, CD-i member Christian Rehberg comments: "We had them in our driving school and there were updates loaded on floppy as new signs, new legal regualtions and such things...."
[Thanks, cdifan, Arethius RGC, Christian Rehberg]