An interesting short clip on why use GMail? According to this video it is 1) Spam blocker 2) Messages stored as conversation 3) Search 4) Gmail Chat.
I have been using gmail for a couple of years now and find 1, 3 & 4 useful, but # 2 is actually annoying. Most of us use 'reply with history' anyways and I find the message stored as conversation, just a nuisance to navigate through. How about you..?