Keep in mind my swivels are 6 pt, you may have a little more options if you have 12pt ones. You can't really use them at 90. But you can get close, if you line up the U joint part of it the right way it has a little give in either direction. Sometimes its just not doable depending on how the bolt is in, that's where the 6pt/12pt comes in. Usually this is just to break loose a bolt that you can't get to easily with anything else. Once its loose, you can use whatever angle you can, extensions, etc.
There are a few places I can think of where I couldn't get an air rat or impact in there, or you don't have enough swing to use a wrench, so I broke it loose with the swivel and then you can use whatever you want. Most of the use my impact swivels have got is at the junk yard with my M12 tools. all manner of collision damage as you can imagine.
Sometimes, you can do the same with a U joint and not need swivel sockets. Sometimes a wobble extension is better, or just as good. Sometimes you come across something where only one of these is any good.
There's not really any right or wrong answer for any of the above. For a DIY guy, if you don't have the luxury of a lift you are stuck doing some things from below and others from above and still others through the wheel well. Pros tend to have more options for this and yet more tools anyway, so there's definitely a place for them.
