Because you can just click the portraits on the left or click the characters in the world to swap which character you're controlling. So rather than pressing skip turn three times to skip your turn for each character, you just press it once.
Those jobs can get you some extra money while you're doing the important thing: winning enough battles in order to challenge the boss and get the reward for defeating him. Once you have all three party members you should be able to smash through the Rats without taking much (if any) damage. You probably don't need a ton of money by that point, but there's no penalty for taking those jobs so you might as well.
