Like you said there are injuries, players carrying niggles where they play on due to a lack of suitable replacement.
Fatigue is a real thing in a position where the player is constantly on the move.
Competition keeps performance levels high.
Players lose form.
The rotation option may have slightly different quality that suit playing against particular opponents.
The rotation option may turn into a better player than the first team option.
these are all suitable reasons to have a high quality rotation option for a full back.