Roll is the first Evasion technique, a long-distance dodge. It is the root of the Evasion tree and your way out of fire and big unblockable swings.
A short dodge won't clear wide sweeps like a halberd spin or a brute's swing. A roll covers far more ground, and it is the fix when fire archers or sappers set you alight.
Every other Evasion technique needs Roll first.