It was the early 90s. Bill Clinton was in office. Eight megabytes of RAM was considered a lot. Most geeks who weren't in college didn't know what the Internet was yet, and likely spent most of their online time dialing into local BBS systems.
CD-ROM drives were starting to become affordable.
So you went out to CompUSA and bought yourself a brand new shiny CD-ROM drive. Upon opening it up you notice there is also an ISA card in the box with it. Yep, it uses a proprietary interface. Long before ATAPI CD-ROM drives came along, you had these beasts which required a special card just to use them. At one point sound card manufacturers realized they could put these interfaces on their cards as an added-value feature.
The Creative Labs Sound Blaster Pro supported Panasonic CD-ROM drives. The Gravis Ultrasound MAX supported Panasonic, Sony and Mitsumi drives, and was one of the largest sound cards ever made as a result.
Eventually the ATAPI CD-ROM drive came about, and we were able to put all this nonsense behind us. But when you see an old sound card and wonder why the heck there's CD-ROM connectors on it, this is why.
(Oh, and goddess help you if you accidentally connect one of these old drives to a modern PATA bus. It's a good way to let the magic smoke out!)