LordKit

I usually order my infantry and spearmen to form shieldwall, or similar and then "stand closer" 2 times.
This gives them a very close formation. And it gives them a good advantage in most battle-situations!

Case 1 - Cavalry is charging them
The horses will be stopped very effectively and the whole cavalry gets stuck in my men's formation,
making those mounted guys an easy kill for them.

Case 2 - Archers/Skirmishers are attacking
The closer formations lead to less space between the shields, so my men are better protected against
the missiles.

Case 3 - Infantry is attacking
The Warband-AI is not the smartest and this usually leads to single men of a big party try to fight alone against
a whole bunch of enemies, because other men of their group are slower or got distracted by other, scattered foes.
If you use formations, your infantry is mostly moving like one man with hundred swords, slashing down the less
dense groups of enemies very effectively.

My archers/skirmishers are usually in 2 times "spread out". This makes them a worse target for enemy missiles.
Then they aren't as vulnerable to enemy fire than the enemies' archers are, which usually stand closer together.
They also leave enough space to let my cavalry charge the enemy through their rows!

My cavalry usually uses the Wedge formation. This way they split up the enemy's rows into smaller groups.
Most times I retreat them after that, letting my own infantry slaughter the scattered foes.

So I have a good use for formations^^