Pro days just give the QB the opportunity to throw to receivers they are familiar with. It helps the QB show off things like timing routes and deep balls where he needs to be in sync with the receiver or know how much speed he has. It allows for a crisper demonstration.

 

The advantages of familiarity with the receiver is actually heightened once they go pro due to much more extensive practice time and the potential of much longer careers together, so throwing at a pro day may actually be a much more realistic view of a QBs ability than the combine.

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