Do people have roommates because they cannot afford the rent on their income alone or because they just want to pay less?

My experience is that both are likely reasons.

It varies. 

My last roommate was a friend that could not afford rent. I got a free 2 bedroom with my job so I let her stay with me.

Both, neither or either, depending on their situation. 

Some also like living with other people, choosing to rent with friends or friends. 

Some prefer the freedom and options available. Students generally live in shared halls as standard. 

No one can afford anything anymore in America. 

There could be a host of reasons for having a roomate.

1) share expenses

2) Lack of availability of other apartments at reasonable rates.

3) Just enjoy rooming with a close friend.

4) Both employed at the same job, during the summer.

5) Transportation.

Yes.  And some people just don't like to live alone. 

they like having them for company