I never understood before why would someone prune a plant. Why cut a branch when you can have the plant grow big and strong and wild? But everyone keeps doing it, so I asked some questions, to see what's up with that.
First reason why people prune is to make a plant grow in direction desirable to humans. For instance, plants like tomatoes and basil will give you more produce and live longer if you prune them. For basil in particular, it will go to seed if un-pruned, and make flowers, then die, because thats it's purpose. But we don't want that, we want basil leaves, to make pestos and spices and teas. So by pruning, we refuse to give basil flowers, and then basil grows more leaves instead. Which is good for us, I assume annoying for basil.
Second reason is to make plants grow bigger and stronger. I've never pruned a tree, but I've been taught that if you cut off lower branches, and don't allow the tree to develop a whole mess of small branches and only let the bigger and stronger ones grow, the tree will focus on growing wider and stronger trunk, and grow higher up. Otherwise the tree focuses on maintaining all those branches and grows up slower. This is great for growing forests and big strong fruit trees, but I am personally annoyed if all lower branches are gone and I can't climb the tree. Very inconsiderate.
Third reason is to save plants. If a part of a plant is diseased, cutting it off can save the plant. If a part is dry, cutting it off helps the plant relocate its resources to maintain the healthy parts. If a plant is growing bare branches with little or no leaves, it's often better to cut it off, because plant spends a lot of resources maintaining that branch, and gets very little out of it.
The last lesson I found out is about why someone would cut the roots. I always thought big and strong roots were vital to life of a plant, and while this is true in nature, it's different if you're growing a plant in a pot. After a while, roots run out of places to go and just end up intertwined, going in circles around the pot, and they're still sucking in water and nutrients, but now these resources have to go a very long way up, to get to the branches and leaves. Tree spends a lot of energy just pushing this water up, and the tops of the plant can dry out.
On the picture above, you can see the roots of a small lemon tree. I've been growing lemon trees for longer than a year, and lately they lost all their leaves, which I was told was normal, but then the tops started to dry up completely. I got them out of their containers today, to check the state of roots, and they were long and messy. I pruned them a lot, then placed them in water to make sure they're well hydrated before going back to the soil. I pruned the tops as well, so now all of the tree is healthy, and hopefully they'll recover.
Holding a tiny tree in your hand is special, you can only do that when they're small like this. They could outgrow you ten times, outlive you by centuries. But now they're only babies, and fit in your hand.