What are all the different cycling activities for?

Yes, that is one benefit. But I also believe it benefits popularity routing data. If people use the most fitting activity profile for the type of riding they do, it is less likely that when you and I want to take a roadbike trip the fenix or the edge doesn’t navigate us through a mtb trail. (When using popularity routing). This principle applies to Trail run vs regular run as well. This is my understanding. I might be wrong. 

I think also some of them do calculate your metrics differently depending on which one you pick - certainly that's true for eBike at least.

There is for example a "cycling to work", but I did not see any difference to the regular "cycling", It doesn't even preset a different "event type" or a different title.

KrapfenKringel said:

It doesn't even preset a different "event type" or a different title.

Yes it does. Gravel ride this morning. I have not edited the title, just used Gravel Bike from the Activities list.

There are different default data screens too for the different bike activities.

And to answer the question 'What are all the different cycling activities for?" They're there in response to people asking for different cycling activities.