On Twitter it is very common for people to follow back the users who follow them – this works to increase their reach, and may attract even more followers in the long run. There are also those who never follow other users back, often in attempt to appear more "real" – they fail to understand that you can't really get more real than real.
It might not matter much when your twitter account only has a couple of hundred, or a couple of thousands of followers – but for people in the tenth of thousands (or hundreds even), following other users back can turn out to benefit their network, especially in the long run.
Why you should follow back other users
Lets make it clear, you do not have to follow back everyone – maybe you don't agree with them on a certain subject, and do not want to follow them for this reason. Not wanting to follow someone, does not mean that you would want to not follow anyone at all!
A lot of companies on twitter fail to recognize the value in following people back, so a lot of users seem to lose interest in following them. Serious twitter users will usually not follow someone who do not follow them back, unless they are of special interest somehow.
Following people back is the first step – it shows that you care about your network. The next step could be engaging with your network – people like to be noticed, even for the occasional casual chat.
But most importantly, each user that you are following is essentially a free ad for your own twitter account – people often check out the followers of other users, or who they are following.
What about fake users?
Fake users is a problem to twitter, they are generally not a problem to other users, unless they are posting a lot of spam. It can be very hard to know if a user is fake, and checking if a user is real or fake can turn out to be very time consuming, especially considering the amount of activity on twitter – so generally, you likely wouldn't want to think to much about it.
See also: Are fake twitter followers useful?
Follow for Follow
You may have noticed certain hashtags such as: #followback and #teamfollowback on twitter, people using them are simply stating that they will follow you back if you follow them – it does not mean that they will follow back everyone. Buying followers on twitter is against the rules, but following people back is not.