Those who start working out tend to forget how they thought about doing their first 50kg in benchpress, now they might be doing above 100kgs. Many Twitter users think the same way about their followers. There are those who thought it was awesome to just hit 500 followers, but now they might be doing near 10000 followers on twitter.
Twitter is fun Because..
To some users, Twitter is fun because, everyone has the potential to become a web-celebrity, and those who are willing to risk that their accounts gets suspended, might also be able to achieve just that.
The best technique employed to get more followers, is to follow and unfollow like crazy. The problem with that technique is that you risk getting your account suspended. With that said however, twitter doesn't appear to look as harshly on people using this technique, then for example Facebook does at sending Friends requests to people that you don't know.
This may be because Twitter is a marketing friendly website. In SEO and Marketing, its like getting people to subscribe to the E-mail list that they normally wouldn't subscribe to, but may very well end up liking without knowing it.
Spam on Twitter
The only reason why Twitter would want to limit the number of follows and unfollows you can do per day, is to lower their server resources. On one hand, Spammers hammering their website with requests can cost them a smaller fortune in hardware and bandwidth. But on the other hand, they might want to keep this web-celebrity potential open to non-spammy companies and users.
It will be interesting to see if Twitter also starts to limit the marketing potential of their service, the same way that Facebook did when limiting their "suggest to friends" feature. Blackhatters would abuse the suggest to friends feature, by sending suggestions to about 5000 friends at a time, either on fake accounts, or accounts they paid to send these suggestions.