A single campaign report tells you how one send did. To see patterns — which subject styles win, whether Tuesdays beat Thursdays, how fast your list is growing — use the reporting tools under Analytics.
Custom reports
The custom report builder lets you assemble the view you care about: pick the metrics, the date range, and the campaigns or segments to compare, and save it. Come back anytime and the report reruns against current data — no rebuilding.
Audience growth
The audience-growth view charts your list size over time — new readers in, unsubscribes out, net growth. It's the single best gauge of whether your forms, landing pages, and swaps are actually working.