Do I need to keep my iCloud subscription if I have enough storage on my device?

Not necessarily but two things to think about. 1) Do you want to sync data (e.g. photos, files, contacts, notes, calendars) between the iPhone and other devices? You may need more than the free 5GB iCloud plan to sync some or all of those things. 2) What happens if your phone is lost, stolen, broken, etc.? Are you backing up or syncing the phone somewhere else (e.g. a computer). If not you would lose all of the data on the phone.