Did you know that Time Machine stores a backup on your local hard drive if it can’t connect to your laptop’s backup drive? Those backups can consume a fair amount of space (say, 100GB or so) over time.
If you use Time Machine, take a few minutes to read this post from Lory Gil. You’ll learn how to disable and re-enable those locally stored Time Machine backups.
And if you are wondering where on your machine those backups are stored, start here. That article is a few years old but, as far as I know, still accurate.