There are many reasons why it's useful for an event organiser to know roughly how many people will join your team. For a start it allow them to estimate how many racers will attend the event, and therefore plan accordingly.

Event organisers may also incentivise racers to sign up bigger teams by offering discounts.

Don't worry, you can always sign up more people than you've estimated, however, if you've been given a discount there may be penalty charges if you do not reach the number of team members estimated.