Answer:
The reason is due to specialization. The baker is going to be better at making certain things compared to the chef (like desserts and bread) while the chef will be better at making certain things compared to the baker (like the main meal). The baker and chef are better at their respective jobs and so, they learn to produce more quickly and with higher quality at their positions. Working together and splitting up the work at the positions they are best at is more efficient then them doing both jobs themselves since they simply are not as good as the other at one specific job.