Answer
The clearest way to see the difference is to walk through two failure scenarios side by side. A disk subsystem fails and the LPAR goes down: with HA in place, replicated data on a standby system lets the team swap roles and have the business running again in minutes, while backup alone means restoring from the last save point and losing everything since that save ran. Now consider a different scenario: a batch job runs wild and corrupts a set of files at 2 AM. HA replicates that corruption to the standby system almost immediately, since it is designed to mirror changes quickly, so the only real fix is restoring from a backup taken before the corruption occurred. Neither technology covers both cases well on its own.
This is why mature IBM i environments run both rather than choosing one, and why buyers should resist a vendor pitch that frames HA as a replacement for backup or vice versa. The practical question during evaluation is how the two work together: does the HA platform interfere with or complicate backup scheduling on the source or target system, can backups be taken from the standby node to reduce load on production, and does the vendor offer both capabilities under one roof or does the shop need to integrate two separate products and support contracts.