As long as a President is more progressive than 217 House members and 50 Senators ...

... I don't see why it would matter how much more progressive he or she is at that point.

There's a limit to the legislation Congress will pass. It cannot be more liberal than the 218th most liberal Representative and 51st most liberal Senator will support. As long as the President is more liberal than those people (for the previous Congress, that's Jim Costa (D, CA-16), and Lisa Murkowski (R, AK)), he or she is going to be pushing legislation that Congress will make more conservative.

I'm pretty sure all of our candidates (except Webb maybe) are to the left of Jim Costa and Lisa Murkowski, which means that the next President's ideology is not going to be what drives policy in the country, just like the current President's ideology hasn't been what's driving policy in the country (though Obama's unilateral executive actions may be changing this question).

Why is it important to people to have a President who is more progressive than the policies that will pass Congress? What does that accomplish?

(Note before straw men come out: I entirely agree some Presidents will be more effective at wrangling legislation out of Congress. But that's a very different question from how progressive that President is.)