Locking on to a target

[Sacred-Oblivion]

I assume you mean "Focus Target"?

This isn't necessarily beneficial during the beginning of the game, but in end game, it becomes vital.
You'll need to generally focus target on the boss (not always) in situations where you'll target something else so you can still see the boss' cast timers and hp (among other things).

I use this in T1 and T5 mostly, but different jobs will have different use for it. Any character or npc can be focus targeted. The possibilities are continuous.

Maybe it's something only on the PS3/PS4. Focus Target is different from "locking on" which annoyingly makes you move slower when moving backwards from your locked target.