All else being equal, people are more likely to follow people who already have lots of followers: there must be something good about their tweets that makes all those people choose to keep following them.

The number of followers is the only visible indicator of the "goodness" of an account. It's similar to karma on Hacker News, except that it's impossible to miss because it's prominently displayed. This is probably intentional on the part of Twitter, to grow their network.

http://buildingreputation.com/writings/2010/02/on_karma.html

Is the quality of the tweets on a particular account not also a visible indicator of the 'goodness' of that account?

Which one is easier to make a short-attention span decision on:

1) This person has made 1000 tweets, let me read them all in order to gauge my opinion of them.

2) OH LOOK, 5,000 FOLLOWERS! Must follow popular person!

Of course you are right but the suggestion was that the latter was the only visible indicator which is obviously not the case.

Humans are sheep. They follow each other. People see a crowd and go to see what's going on.

You can test it. Create a small crowd in any high street, stand back and what it grow.

Adding to what the other said:

1. Some people get paid for sponsored tweets. The more followers you have, the better chances to get sponsored tweets and get paid more

2. Some accounts belong to websites. These webmasters get lots of followers and show to potential advertisers how popular they are.

3. To game search engines

4. To get some cash when selling the Twitter account with or without associated website