If you have trees on your property, it is your responsibility to keep them trimmed. As trees grow and develop branches and develop more weight, it becomes harder for them to bear their own load. Additionally, these trees become more at risk due to weather conditions and such, so trimming them is essential.
Typically speaking, trimming should be done in order to relieve stress on the tree while still meeting an aesthetic function. That is why it is best to trim the trees in such a way that it fills a purpose while still allowing them to look good. The other reason why trees are trimmed is for their own health. Since every part of a tree takes resources to grow and keep healthy, every extraneous part that does not have to be there can take away form the whole.
Generally speaking, people are going to want to trim lower on their trees, cutting off branches that are closer to the ground. Cutting them so that the line is flush with the trunk will make for an attractive looking trunk that is more effective at what it does. When it comes to branches that are higher up, you are going to want to cut them so that the primary branches are left intact, while getting rid of the offshoots that add a lot of extra weight.
Depending on the species of tree and how big they have gotten, branches adding weight can seriously affect the ability of the tree to develop. Therefore,it is a good idea to relieve the weight in order to add more to the assets the tree has. Lastly, consider that trimming trees makes them much safer to deal with, especially when it is windy. Relieving some of the weight will make the tree more able to stand up to the elements.