For StaffTraveler to work, we need people who respond to incoming requests. To motivate our users to do so, we give them a credit when they submit the loads. The member who posted the request pays for this credit. You could see it as a way of thanking someone for the time and effort it took to give you the loads.
The second reason we use credits is to prevent system abuse. By limiting the number of requests a user can make, we create a better environment for both users and airlines.
You can earn free credits by contributing to the community, or buy credits using an in-app purchase. Credits do not expire.