Backups & restore points · Time Machine and backup strategy
Do I still need a backup if my files are in iCloud Drive or Dropbox?
Short answer: Yes. Sync services copy deletions and damage everywhere; they aren't a versioned backup you control. Data Backup adds a separate history of the folders you care about.
Yes. iCloud Drive, Dropbox and similar services sync — they keep your files the same everywhere. That's great for access, but a deletion or a damaged file syncs too. Most offer a limited way to recover deleted files, but not a long, versioned history you control.
A backup keeps earlier versions somewhere separate. Data Backup can add that history for the folders you care about, on a drive or another account. See iCloud questions for how iCloud storage itself works.