Not 100% necessary these days. Most software and media is available for download/streaming. But Dual Layer DVD drives are cheap. They support everything but Blu-Ray discs.
A Blu-Ray drive will support all lesser forms of discs (Blu-Rays, DVDs, CDs)
Why cases come with 4 bays:
Most ATX case designs come with multiple 5 & 1/4" bays to give an end user options. Back in the day, Floppy Drives, Zip Disk Drives, Tape Drives (for backup), were quite common. There are also adaptors to put 3.5 inch drives in 5 1/4 bays when people use a lot of drives.
Modern users might use them for water cooling reservoirs, pumps, fan controllers, professional audio (1/4 inch jacks for speakers/headphones/volume knobs/equalizers/input selectors), LCD displays for various system information are available.