The same question can be applied to many activities / hobbies. For example, what is the point of standing in a stream with a fishing rod at 5.00 a.m., and then throwing back into the water anything that you manage to catch. Or what is the point of spending lots of money in pubs filling on end with liquid that later comes out of the other end, and leaves you with a headache, damaged liver, or worse.
In most cases, the answer is that participants find it relaxing, entertaining, enjoyable. etc., and better than festering at home watching daytime TV.
In my case, it probably started with a fascination for steam locos, with visible working parts and the visual impact of moving clouds of steam / smoke. And, when I could afford, the opportunity to travel on as many lines as possible, visting, or passing through places that I would not otherwise see. And when regular steam finished, I decided that riding behind /photographing diesel & electric locos was more interesting than stopping indoors.