Battles purpose was to prevent siege.
“the battle is, in this equation, a ‘second order’ concern: merely an event which enables (or prohibits) a siege. As we’ll see, sieges are quite unpleasant things, so if a defender can not have a siege by virtue of a battle, it almost always makes sense to try that (there are some exceptions, but as a rule one does not submit to a siege if there are other choices), but the key thing here is that battles are fundamentally secondary in importance to the siege: the goal of the battle is merely to enable or prevent the siege. The siege, and the capture or non-capture of the town (with its role as an administrative center for the agricultural hinterland around it) is what matters.”
https://acoup.blog/2021/10/29/collections-fortification-part-i-the-besiegers-playbook/