Dara was a fort right? It would have been easier to defend from it's walls rather than meet a numerically superior army in the field.
Dara wouldn’t have been big enough to shelter the whole army. He thought it better to meet the enemy with his whole army than split it up between a garrison that would have had to weather a siege and a smaller field army that would have stood less of a chance against the Persians. Turns out he was right.