Why is it necessary to have starters in an electric motor?

A starter or contactor is used for a couple of reasons. It allows the motor to be stopped and started from remote locations. Incorporated into the contactor is usually an overload device to protect the motor from overloading. Also a contactor is used for under voltage protection. If the power goes off when the motor is running the contactor has to be restarted manually again when the power comes on again. This is two fold protection in case someone is working on the equipment when the power is off and forgets to lock the device out and it restarts automatically. The other part of this equation is that you don't want all of the motor loads starting at once on power return. On big factories this could cause a voltage drop all the way down the power authority's line. They don't look favourably on things like that.