Why do people add you as a friend if they're not ...

I think its much easier for people to approve friends than to actually write a message. Sometime people approve friends when they don't know them (and don't want to know them) but they don't really want to interact with them. Kind of warped but very common.

Well, there are two different cases we are dealing with.

1) Someone is simply obsessed with the number of friends and simply wants to have a bigger friends list.

2) Deliberate Social Media Marketing strategy based on community building.

In reality these two are clearly distinguishable and it's easy to see if a community actually created to promote a third party website (and I believe there is nothing wrong with this).

If you wish to know how community building works from SEO/SEM perspective just ask me a question :-) 

This is very common in social networking sites, where they try to network with people or show more contacts in their contact lists.