If you don't get to the point quickly, people might think it doesn't really apply/matter to them and ignore it.
This became very obvious to me when my day job for a few years was responding to customer service requests over email for World of Warcraft. I would often find myself skimming all the useless (quite literally) pretext as quickly as possible scanning for what their actual problem was.
Stereotypical example of a poor email from a customer:
> Last night I finished the raid with my guild where we downed the Lich King. Then this morning I went to school where my friends and I also talked about WoW, then when I got home, everything seemed normal, I turned on my computer, logged on and entered my password, but it didn't work, then I went to the website and used the password reset, then I tried to log on and it said my account was locked, then I checked my email, and it said my account is locked and I need to contact Blizzard...
At which point I stop reading and I'm thinking "finally, I see why he's emailing us".
To be fair, these emails are often from adolescents who understandingly do not yet have the experience to do effective communication (which is actually an additional interesting aspect of customer service for a computer game compared to services which are only taken up by adults, but I digress).
I now work as a software developer for a startup and often have to interact directly with clients, and when I communicate with them, I always make sure to have my desired "call to action" (even if only summarised) within the first sentence.