Why is my MobileSync>Backup folder so large if I backup to iCloud, and can I delete it without repercussions?

MobileSync/Backup/ contains backups of your iOS devices, if you delete the content you lose all of your local iOS device backups.

It is up to you if you need local iTunes or iCloud backups. iCloud does require an internet connection, so restoring will usually take longer.

See this from Apple…

Find and manage your iTunes backups - Apple Support

At one point in time you did backup in iTunes to this PC. If you didn't backup this device you may have done it with another one.

When you sync to iTunes it normally does a backup up the device.