Time Machine is filling up my backup disk
You paid for a 750 GB backup drive, so Time Machine uses all the storage on it. If you used it for say 5 years, and you still have 86GB free, then every version of every file that ever was on your Mac is still in your Time Machine backup somewhere.
When all the empty space is gone, Time Machine will delete the oldest backups until there is enough space for the next backup, and from then on your backup drive will always stay almost full. So you can restore older files for many years.