I noticed that I was freakishly low in disk space, so used DaisyDisk to identify problem directories.
Turns out, a major offender was ~/Library/Application Support/MobilSync/Backup, which contained several years worth of iPhone and iPad backups.
As tempting as it might be, don’t just go deleting the directories inside it.
Instead… use iTunes, which will keep all the internal bookkeeping correct.
Open iTunes / Preferences…, then go to the Devices tab. You’ll see the name of the backup, an icon if it’s encrypted, and a date/time stamp of when it was made.
Select one or more of the items and press the Delete Backup button. But beware, they won’t really disappear until you press the OK button. Closing the dialog via the red window button or pressing Cancel will not commit the change.
This is one of those rare cases where Apple dialogs don’t reflect current state, but intended ones. And, considering this is the safer way of dealing with data, I’m all for it.