Twitter hashtags

Hashtags are used on Twitter to mark a topic, so that people can follow a group of tweets on the same topic by various people.  So, for instance, this one is to make the tweet stand out to people who are searching for tweets about language:

[video & article] Children developed own #language in cave http://bit.ly/bnKJDL 

But people are using them for other things, where it's clear that it's not to identify a topic or theme.  Here are a few examples: 

You know when you've worked really hard revising a story, then suddenly realize you were revising an old version? #pullhairoutofhead #cry

 Me: "I'm not accepting new work now." Would-be client (self-pub author): "Well, can you just review what I've done myself?" #no

 ugh, the intros are the hardest part. Spent three hours on 3pp writing. Still not sure if it's too-detailed or not-enough. #onward

 SO many ways to get hold of a book these days. I mean there's really no excuse http://bit.ly/myfryapp #shamelesswhoring

So, why and how do people use hashtags?  What does it serve to put it after a # rather than just saying it?