Interface, wiring-Do I really need one?

Hello, first time posting here, longtime lurking.

I have a 2011 Chevy Silverado 'stripped'-no Bose, no OnStar, no steering wheel controls-just an AM/FM radio and 4/6 speakers(if you count the A pillar tweets). I am installing : Alpine IDA305SBT, Kenwood Exelon X600F, Infinity Perfects, etc. What will this $100(+) interface do for me?

Apologies in advance if this issue has been posted already.....

Thanks in advance for advice.....

Chuck