In my Plant DOL starter in 75 KW, RPM of 2975, Delta connection Motor and 45 KW, Rpm of 1470 motor in star & delta starter?

we are using 75 KW motor in a Main oil pump for generator and 45 KW Motor in belt conveyor. belt conveyor load is heavy to take full Rpm and stabilize the load. ok. Then Why is 75 KW Motor using Dol
its amps is 124 A in delta connection the motor amps will increase 6 times so why we use dol.
To all respected senior sir's plz give me clear explanation with formulas.