[markmywords]

Another thing to take into consideration is how prize money is awarded in track and field competitions: There is most of the times a separate amount of prize money awarded for breaking a world record as all hosts of competitions would love to see new world records set in "their" events. This prize money is often higher than the regular prize money for winning.

So for an athlete it makes kind of sense to try and set new world records in incremental steps in order to earn as much prize money as possible. It also might lead to more frontpage cover stories in the long run.

That being said, this might not 100% apply to Usain Bolt as he is already a popular figure contesting in a very popular discipline of track and fields but it might make sense for an athlete of a less popular discipline like Triple Jump.

[Danny72]

It's generally acknowledged that Yelena Isinbayeva has been doing exactly that in the pole vault over the last few years.

[dagw]

And Sergei Bubka before him. Bubka set 18 indoor and 17 outdoor world records in the space of 10 years, often beating is old record by only 1 or 2 cm. There is no way he would have jumped like that if he didn't get a bonus for each record he set.