1. The mailing function is very practical: you can send items to different characters under the same account (so it's convenient for transferring items from a main character to a lower-level one), and you can use express mail so that the recipient doesn't need to go to the post office to pick it up.
2. You can change your job at level 9, and after changing jobs, you will directly reach level 10, so there's no need to gain experience between levels 9 and 10 for leveling up.