I think they are important because I sometimes can't even tell who's the rapper in the group 😭 A group in which I can't recognize where the rap part is, it's Enhypen. It's not a bad thing but I can't really tell who's the rapper in the group and sometimes TXT.
But the fight begins when they add the 'main' and 'lead' things.
Being a main vocalist doesn't mean the member is a better singer than the lead vocalist in the group, I bet they just choose it because the member's probably more skilled, have more experience or their vocals sound more pleasing?? I have no idea lol.
I want to add that the main vocalist doesn't have to be everyone's favorite vocalist in the group and sometimes these 'main' and 'lead' things can be frustrating because if another member doesn't have these positions and it's only a 'vocalist', his/her talent might be wasted in the group :/
I can't say members who have main and lead positions equal to having more lines in a song or leading dance breaks when it comes to dancing (although the chances are higher) because Jungkook and Jimin lead BTS's dances more than Hoseok does and he's the main dancer of the group + in charge of performance. Another example is Lisa being lead rapper but sometimes having more rap lines than Jennie (who's the main rapper) so it can vary sometimes.
If all the members are vocalists and rappers like BTOB, props to them :D