Muser is an Aspiration Trait. All aspirations give your sim an extra trait so that they can complete their aspiration easier. Mostly this aspiration trait will give your sim certain moods or desires when performing a task related to the aspiration Creativity.
Sims that choose an Aspiration from the Creativity category will get the Muser bonus Trait. This trait gives sims better skill gains while Inspired.
Getting inspired is very important in the sims. This way you can create and perform even better. Some jobs also require your sims to be inspired to be able to be given a promotion.
Artists are very much what gives the Sims 4 life. Being able to play the piano, guitar and other instruments may earn you tips from other sims, because they are desperate to see some action.