1) It’s easier to port to another computer to restore and / or copy data.
2) If the PC is physically damaged (fire, flood, theft), your backup external drive can be safely elsewhere.
3) Your portable drive can be used for other purposes.
4) Your computer doesn’t have to be opened, HD removed and reinstalled, and new PC configured to get the backup data.
5) External HDs can be quite useful for those who do multimedia editing and need a large amount of swapable storage.
6) Many external HDs can be connected to the router directly and configured to back up critical data for all computers within a household.