There is always a risk shooting expired film, so why would someone do this? As far as I can see, there are only a few reasons why someone would do this. The first one, is down to the unpredictability. Some people call it experimental or abstract, but I think the general idea of having the outcome be more unpredictable means you are not concerned about how the photos come out, but rather allows you to just shoot for fun.
The second reason for doing this is the cost. Expired film is a lot cheaper. This is a great way to get into shooting film, as expired film tends to be a lot cheaper. You tend to find some people starting out do this, or if they want to just shoot anything, or even just testing out to see if a camera works.
Is it worth shooting expired film? Yes and no. If you are after more consistent results, and have a certain expectation to nail certain exposures then this is not for you. You would be better off picking up a standard film to shoot or a pro level film. For some people, they only shoot expired film, as they want the unpredictability that this provides.
It is certainly worth trying the experience a couple of times if film photography is something you enjoy. You may be pleasantly surprised with the results.
Shooting film costs money to develop and convert to digital. This does mean that a person is less likely to choose to shoot expired film rather than in date film, unless they are deliberately trying to get the unpredictable results.