First, what the $#!& is an optical drive? This term refers to a drive into which you load CDs, DVDs, or Blu-ray discs. "So a DVD player?" No, an optical drive.
A lot of people remember when DVD was the most common way to watch videos, before streaming. However, software publishers also sold their stuff on DVD-ROM. Believe it or don't—the very first version of Skyrim was released on DVD-ROM. Microsoft Office used to come on DVD-ROM, and before that CD-ROM. How old is the CD-ROM standard? Old. Late 1980s.
That's why they're called "optical drives", because that's easier to say than "CD-ROM/DVD-ROM drive", even though that's exactly what it is. Modern ODs can usually also read Blu-Ray discs.
Newer PCs these days are being built without ODs, and that's a big problem; especially if you want to use discs to store files or run old software. An easy alternative to having an OD installed is to get one that connects up to the USB port. Like I said before, literally everything that has a practical function connects to a USB port.
Explicitly, why do you need an OD? Because of the vast array of media possibilities that exist for people who are able to load disc media on their computers. File storage, video DVDs, music CDs, and nearly all the software ever made for Windows 98 through 7.
Some ODs are tray-loading, like your grandma's Windows Me computer; you press a button and the tray pops out for you to load a disc (tray ODs designed for use with laptops will have a spindle in the middle that clamps the disc into place to prevent it coasting about during transport). Others are zero insertion-force, or slot-loading, like the Nintendo Wii and WiiU; insert the disc into the slot and the OD will pull it into itself.
To use a disc, place the disc onto the tray or insert it into the ZIF slot; either way, make sure the reflective side is facing down. If using a tray-loading drive, close the tray. The PC should recognise that something has been loaded into the OD and will ask you what you want to do with it; usually the options are to run any Autoplay process the disc has, prepare it to have new data written to it, open the drive's file directory, or cancel without taking any action. If you're installing software from the disc, select the Autoplay option.
If the disc is blank, Windows will ask you to name the disc and decide whether you want to burn media to it (i.e. write data to the disc permanently so it can't ever be changed) or use the disc like a USB drive, jockeying files around between computers and changing the contents of the disc. If you're making an audio CD that you want to play in a dedicated CD player, you'll want to burn files to the disc. If you're just making an archive copy of your N64 ROMs folder or something, use the disc like a USB drive. "Burning a CD" sounds dangerous, doesn't it? It isn't, honest. It's just a computer term, like "paste".
This next bit is perhaps the most important, yet the most ignored. Do Not Ever Touch the Reflective Side of a Disc! Read that again, out loud this time. The oils in your skin will cause the data surface of disc media to deteriorate over time until the disc becomes unreadable. If a bit of dust or something lands on the reflective side, brush it off with a dry cloth; if liquid gets onto the reflective side, lay the disc onto a dry cloth and let the liquid absorb into the fabric. If the liquid was anything other than plain water, make a copy of the disc as soon as possible, especially if it's an archival disc, because most liquids are acidic or basic enough to cause a reaction with the adhesives that hold the disc together.
Another problem is clouding and oxidation. If you're certain that your disc's data surface is clean, but computers or other devices are having a hard time reading from it, check the data surface for cloudy, swirly, or fragmented patterns. This is especially common with older discs, say from the '80s to about the turn of the millennium; companies hadn't quite gotten their adhesive formula worked out then and a lot of them break down. Also, the data surface should be clear, like a mirror; it should have no colour to it. If it's brown or yellow, this is an indication that the adhesive has oxidised. If the disc is still readable in a PC, make an archival copy of the disc as soon as possible.