There are several reasons why you would want to apply throttle before the apex, but the very important point before considering any of this is that adding lean angle while accelerating put a lot of stress on the tire. In this situation the tire can react unpredictably, losing and regaining grip uncontrollably and shaking the rider out of the bike.
At any rate, when giving gas in a turn we transfer the bike weight to the rear making the front easier to handle. In a chicane, peaking up the throttle in the middle of the turns helps with the fast direction changes (but definitely not while giving the first strong lean input). Also when riding on bumps ideally you want to open the throttle, for the same weight transfer reasons.
Another reason is engaging traction sooner. Racers tend to give some gas before the apex to start engaging traction and power up immediately after the apex. This is a lot more obvious in flat track or dirt riding when riding through the turns with a smaller displacement bike.
Finally in very long turns we have no choice unless we want to park the bike somewhere in the middle of the turn.
I prefer the expression "picking up the throttle" to "maintenance throttle", because it the former gives better the idea of a minimal input.