Sage Mode doesn't necessarily grant you anything new. It basically just enhances what you already have. I like to think of it as like the "Super Saiyan" of the Narutoverse.
Imagine Naruto in his normal form puts his Rasengan against Kakashi's Rasengan. Neither one is doing better than the other, and it's a stalemate. But, for some reason, Naruto manages to enter Sage Mode and they try their regular Rasengan's again. Now, Naruto has a stronger Rasengan and he'd overpower Kakashi even though it's more or less a "regular" Rasengan.
Hashirama and the others could likely do lesser versions of their jutsu without Sage Mode. I mean if you compare the Giant Buddha Technique of Guruguru who wasn't a sage vs Sage Mode Hashirama's Giant Buddha Statue, you'd see what I mean.