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
Last, but not least (in fact most recently) is travel. As a side effect of getting a summer house in the mountains of Eastern Europe, 7 years ago I did a run with a car across Europe to get some luggage to it. To be more exact it all started with that unpronounceable volcano eruption in 2010 (Eyjafjallajökull). While being stuck in Eastern Europe with no flights, I ended up doing the research on the "immigrant's run" and preparing to do it. While I managed to grab the first plane out instead of driving in a rusty old banger cross Europe, the idea did pique my interest so we tried it with a proper car in the other direction. The first time I did it, this was a distinctly "immigrant's run" experience doing exactly what the gastarbeiters were doing. I hated that, but I did not abandon the idea. It took me a couple of tries to figure it out. It is, in fact, quite pleasant if done right - stick to under 8h per day driving and always finish in a classy mineral water jacuzzi. There is a "Therme" in every second village in southern Germany west of the Rheine valley and south of the Frankfurt-Dresden line. This glut of places to stay continues east all the way to the Czech/Polish border and south through Austria to Slovenia and Hungary. So if you want
to make your travel pleasant, you actually can.
Places like Bad Kissingen, Szeged, Gyor, Teplice, Cochel, Badenweiler, etc will take care of that. In order to fit with this lifestyle I am now onto modding a third car so it is comfortable and has the relevant amenities for long range travel.
In any case let's get to the point. This is organized by topics: