Some people do it for fun. Some people do it for cost saving reasons. I personally do it in two cases:
When nobody will do what I want. An example would be additional electrics to make a car geek friendly or squeezing the last inch out of available space in the house, garden or elsewhere. Another example would be getting a garden outbuilding properly damp-proof and rodent proof so it lasts for decades even in the English climate. A variety of the "nobody will do what I want" use case is figuring things out which nobody knows how fix in the first place. In that case it is not that people do not want to, they just do not know how to so you end up doing it yourself by trial and error.
To avoid dealing with contracting logistics. If you want things done properly you often need to get 5-6 professionals involved and you neither have the time, nor the patience to organize that. A classic example would be refurbishing a kitchen without taking it out of use. The UK tradition commands that you are supposed to live on takeaways for a month while you are waiting all the muppets who are supposed to be doing your new kitchen to take their turns. That is a bit difficult for someone who cannot just eat everything
In any case let's get to the point. This is organized by topics: