Why in power stations Battery charger is used to supply Dc source to Switch gear when you can have AC control devices instead ?

DC control power is supplied to switchgear so it can be backed up by a battery. You could use AC power and take the DC through an inverter to have a battery back-up, but inverters can fail.  Making the function of switchgear as fail-safe as possible is important.  There are huge currents running through switchgear and being able to interrupt them saves damage to equipment.  Notice that molded case circuit breakers that handle smaller currents normally supply their own AC control power from a transformer on each MCCB.

Power stations typically have a standby or emergency power system so they can control all of their equipment in the event of a power failure. It can be all DC or a combination of DC and DC inverted to AC. In any case the basic power source is a bank of batteries.

Presumably so the switchgear can still be operated in case of a system shutdown, when no AC supply is available.