First Pistol Build: Tips for installing Flash Can

[buffdriver]

I purchased a slim profile KAK Flash Can to install onto an Aero Precision 300BLK upper. In reading, the purpose for putting a crush washer on before the Flash Can isn't clear. What does it do, and more importantly, is it a required item?

Timing is the process of lining up the Flash Can to be oriented a certain way when the Flash Can is tightened. Is that accurate?
Therefore, since the Flash Can doesn't have any ports as with an A2 flash hider, what purpose does the crush washer serve if it isn't needed for timing?

Thanks.