Let's say you do trucking. Each time you complete a delivery you get 5% bonus in cash and score (I know score is hard because small numbers, could scale by 1-5 or something) for certain amount of time, It can scale up to let's say 50% If you are doing the mission regularly (Like let's say you are dedicating whole gameweek to trucking (If you aren't doing the mission for let's say 15 minutes the bonus is gone).
I personally won't use It but It will clearly encourage people to do missions, as these pay little amounts and are boring.