Is there a purpose for attendances?

As far as I'm aware, yes. The more attendance + revenue = More $$ for CS.

I think it impacts home field advantage.

Yes I think JD said that it has something to do with crowd noise.......I think.

Both answers are correct.

Crowd noise?

Somebody say "Crowd Noise?"

How do you get your attendance up? My teams has won 21 straights games dating back to last season and we still do not sell out our games? Is it time to move to a new city? If so, will my attendance improve as a new city would be excited to have a new team in town?

The location doesn't impact the attendance, only your recent success - your attendance is probably representative of the top level. The chart is showing attendance and not ticket sales (that actually might be a good metric to add, I'll put it in the idea list) so it is taking into account the no-shows which rarely if ever are there 0% no-shows.

But wouldn't you think a league champion that has only lost 2 times in a year and a half would have attendance in the sold out range? Not 4,000 no shows for a home game against a 1 loss or divisional opponent. That would never happen in Green Bay, Pittsburgh, or Dallas...Plus teams that are usually just short of a sell out have deals with local corporations or sponsors to buy up the last couple thousand if needed. It's a very small thing, but I think the numbers need to be bumped a bit. We are the defending champs on a long win streak, our house should be packed. Even if we lost a game it should take a couple of weeks for the attendance to go down. Just my 2 cents. Minor thing, I love this game!

Actually Green Bay averaged about 2,000 no-shows in 2014, which at a glance without actually running the numbers seems to be close to what you're getting. You had an attendance drop for the last 2 games but it was probably the result of a dice roll as opposed to anything else.