starter relay needed? ????

It is hard to explain, but I will put down a few things for you to do additional study on by looking up and reading about.
A starter solenoid is an inductor. It draws several amps. it slides the bendix, and closes large contacts that turn on the starter motor. To turn on an inductor is easy, the current starts at zero, and builds in time to what is limited by the circuit resistance. You can measure the resistance of the solenoid and use Ohms law to estimate the current.
The interesting part is turning off the solenoid is the hard part! When that happens the current is high, the inductor stores the current, the voltage increases, the switch contacts may weld, keeping starter engaged. It is the inductive kick, similar to the operation of ignition coil.
So that is a reason to use a relay that is designed and rated for the purpose.
A relay coil typically draws tenths of an amp, the circuit is easily wired with a NSS as an interlock at the low current.