One thing you can do after winning battles is to loot the enemy settlements. Looting is what you do when you are not interested in long-term gain such as taking control, but in short term gains, in other words: plunder.
When looting, you decide what your aim is. Some of these are only available when you are inside the settlement, while the others are more effective. When you are taking spoils from some outlying farmsteads, you simply don't get as much.
You can simply take valuables and coins. This will give you gold into your characters purse, but with a great inefficiency as your men will also keep a lot to themselves (i.e. the loss to the settlement is much higher than your personal profit).
Another option is to steal resources, i.e. food, wood, metal or goods - and send them to any estate of your choice. There is a small chance the lord of the settlement you plunder finds out through rumours or spies where you sent it. So be aware of being found out if you let other people loot for you. Then again, it opens the way to false flag operations...
Instead of stealing things, you can also steal people, throw them into chains and sell them as slaves. This works the same way as resources (select a destination, small chance of being found out), however the destination also needs to allow slavery or you can't send there. Some realms just don't do the slavery thing.
You can steal resources, such as weapons, armours, horses and other goods, provided you have camp followers set to collect them. Basically, an instant refresh.
You can also (additionally or just) simply destroy, by burning down the buildings in the target settlement. This will damage them and make them inoperable after some damage. However, the previous workers of the building will automatically begin repairs, so it will come back after some time. If you want to destroy something completely, you have to stay and loot again, preferably with several people.
As with to many things in this game, you can loot every settlement - enemy, friend, even your own.
No, there is nothing wrong with looting your own settlement. When you loot your neighbor's it's looting and when you do the same thing to your own it's taxes. Taxes are always ultimately collected by force anyway. That's why they aren't called donations!