Caddies were fairly common in early CD-ROM drives. Tray-loading (and, even later, slot-loading) drives were a later development.

One theory I've seen is that caddies were developed in part to protect valuable data CDs from accidental damage, and faded in popularity as software became more affordable. Early multimedia software could be quite expensive, with some titles running into the hundreds of dollars.

Discs always used to be in cases/sleeves. The caddies were a natural extension of that same metaphor.

5-1/2: "floppy" plastic outer shell with a rectangle cutout across the disc, and a circle cutout so the disc could be squeezed/grabbed and then rotated. Stored in a paper sleeve to protect from scratching, all those were usually in a plastic case that held 10-100.

3-1/2: hard outer shell, metal exposed ring/hook in the middle, spring-closing door to protect from scratching. These had gone from ~360kb to 1.44mb (4x increase) and space hadn't bloated out yet. They were durable enough not to bend, and the protective door meant it was semi-dust/sand-proof.

Then along came CD'd... jewel cases, but you're carefully handling the actual media (ie: that magnetic disc/vinyl "record" from within the 5-1/2 floppy).

Caddies gave you the feel and protection of the 3-1/2 hard case disks, and were actually pretty useful if you had like a 6-CD encyclopedia set (eg: Encarta 2003 - https://news.microsoft.com/source/2002/06/27/microsoft-encar...).

You'd generally install a 50-100MB program and have to swap CD's depending on what program you had open (or what it was asking for). Even! There were IIRC 3-disc changer drives (like car audio) where you could load up a cartridge and switch (slowly) between discs 1, 2, and 3.

In some cases they were really useful! We had one with like a 20-slot Rolodex style storage box and you could load up the caddies (and type labels!) and keep the optical media safe from grubby kid's hands.

Zork, Myst, 7th Guest, Encarta, Clip Art bundles, font bundles... at a time when Nintendo was the contemporaneous technology, switching "cartridges" to whatever you were working on was an incredibly efficient use of space and money compared to how expensive hard drives were!

The caddies were just a simple loading mechanism, with a spring door like a floppy disc. I suspect they had the life they did because someone was hoping that we would all buy ultra-expensive caddies for our collections instead of moving discs in and out of cases.