If you hover over an army, you can see what they're doing.
They always have a purpose, even if you don't understand what it is. Sometimes that purpose is out of your line of sight.
If they're "going to a point" they're usually gathering up parties - but those parties might be delayed by looters or an enemy party or some other engagement, or defeated in transit - so it might appear like the army is wandering without point - when in fact they're waiting.
There are a number of reason why an army that is attacking might stop and head off in another direction - that their original target might no longer be suitable, it might be taken, or sieged by another army, so they stop suddenly and head to another target. Or their original target might be too strong, so they stop suddenly and head off to find another one - this is often the case if you're lingering about - you're the thing that makes an average city, too hard...
Or the army might suddenly be tasked with defending one of their cities that has come under siege - only for that city to survive the siege, or for another army to get there first - which would definitely lead to seeming like it is wandering about. They might walk for 4 days in one direction to find their mission change, and walk 3 days in another direction.
These effects are exaggerated if the AI kingdom is large, or dispersed - armies can run out of cohesion while being in transit - stop, and reform with another leader... This is the nature of the game - with constant events happening in real time. It can seem like everything is just random wandering, when in fact the AI is responding to several different kinds of crises at once, that are constantly changing separately.
Just a few guesses. But the because the behaviours are scripted, they're always logical, even if you don't see the reason.
Grand strategy to effectively utilise the armies on the other hand... is something that the game needs.