Red, green, yellow, black wires in a cable make this "telephone quad cable," almost definitely used at present or in the past for landline telephone wiring.
Why's it there? This was a good place for a former owner/technician to put a junction. May have been easier to pull from both ends to this box, may have needed to tie in a line nearby and wire already ran through here.
What to do about it now? I recommend leaving it. Landlines aren't used nearly as often now, but are still present in nearly all older (and some newer) homes. The wiring can be repurposed in surprising ways, like carrying signals or voltage to security systems and smarthome devices. You can repurpose this wire for one of those uses if you have smarthome devices to place near there.
Alternatively, you could pull it out; but you'd likely want to fix that hole after you did, and it'll be difficult to get it to look good (smooth) without replacing that entire board.
As some others have commented, feel free to get a smaller cover plate if it'd look better to you. Although I think the reason that one's there is to cover up some sloppy cutting around the edges of the hole.
