Impressions work for brand building. If you're Coca Cola, Mars, Frito-Lay, etc... this is meaningful.

But if you're Ferrari, then you want some segmentation, rich old dudes and young kids that might be rich in the future.

Going even further, if you're a small business, you want further segmentation based on locality and possibly other demographics.

AFAIK, twitter sucks at this audience segmentation for advertisers. Facebook does really well at this, but their audience is shrinking and getting poorer.