Not the enemy units but your units. Sometimes your units take a detour or are standing at a tile which is tacticalwise less favourable. In one battle my champion moved towards the enemy unit like this:

starting side player:          Champion--> enemyunit        enemy starting side

My unit had plenty of movement to move to the backside of that enemy unit. Because it choses that side I had to spend another turn to reach the other enemy units (which were archers). If the player could see and choose the attacking side (like below) it would be more tactical:

starting side player:                     enemy unit<--Champion     enemy starting side

There's actually a 1 tile ZOC around each enemy unit that you can't simply run through (otherwise fast units would have way too high of an advantage).  It's the reason you run around the enemy in weird ways sometimes, but we do a really poor job of explaining the system exists  But 'Visible tactical ZOC' is on our list.

I always move and then attack separately, so this has never been an issue for me.  I can see how it would be useful, though.

But, if positioning is important, than make sure you move and are positioned correctly first, then attack.