I think it depends a lot on the type of comic. Generally you only need as much background as is required to tell the story or joke that you're telling. Generally humour comics need less background because it's pretty rare for a joke to stem from the place so much as the characters (not completely out of the question, but a lot less common).
Story and action comics, on the other hand, may need backgrounds for:
- Setting up the world the characters live in. What is the culture? What is the tech level? What is a person's life like in terms of wealth and status in that world?
- Establishing tone. Sunny and light? Grungy and dark? Cold? Warm?
- Showing where characters are in relation to each other. ie. Is MJ at the top of a burning building while Spider-Man is at the bottom!? Is a monster blocking the only exit in the room!?
Like on my latest page:
I put in a fair bit of background drawing because I needed to firstly establish a change of scene from the previous page, we're in a different place with a different character, but it's still the same light and weather conditions outside, so she's probably not that far away.
In addition to that, the environment also does a fair bit of heavy lifting when telling us about this character. She has a big desk that evokes authority and leadership, the room is quite large and empty to give the sense of isolation and loneliness. The furniture is Victorian-period and made from dark coloured woods, which feels dark and again, authoritative, as well as supporting the idea that this is an older character with an established fortune and a sort of faded grandeur. Finally, there's a lot of blood reds in her environment to... well, I won't spoil anything about that. 
I built out the whole room in 3D to allow for interesting camera angles while keeping the layout consistent and help me with all the perspective. It was a lot of work, but I feel like it was worth it.