Whenever there have been talks about getting a new crew member among the Straw Hat crew, we find Luffy fixated about getting a musician; rather than getting a doctor for medical emergencies which should be their first priority since there is no end to the fights, the crew will get into.
A few of the reasons that were consistent and rational are going to be discussed here.
Some assume that Luffy’s obsession with wanting a musician might be Oda’s way of foreshadowing a new crew member. Eiichiro Oda is a master of subtle hints and a cryptic way of storytelling for which One Piece is known for.
Luffy repeatedly mentioning how the crew needs a musician might be Oda’s way of foretelling that a musician is going to join the crew momentarily.
There is also the fact about ‘Bink’s Sake’, which is one of the most notable songs that was sung by the Roger Pirates as well as Shanks’ Pirates. Therefore, having a musician who can sing Bink’s Sake in the Straw Hat crew serves the purpose.
Personally, I believe that we have not heard enough of Bink’s Sake and it could possibly have even more significance in the future when the One Piece is finally discovered (because we know from White Beard that “The One Piece is Real!”)
Another reason for this can be to poke fun at the fact that how real-world pirates were also obsessed with musicians, more than they were with acquiring actual treasure.
Having a musician on board was quite popular back then and was considered an important addition when pirates set out for sailing.
It is understandable how good music along with lots of food and booze is one form of entertainment that pirates can have.
Luffy wants a Musician in His Crew? might be Oda’s way of referring to real-world pirates and at the same time trying to make it as realistic as he can.
A different way to look at this question is when Luffy is asked why he wants a musician and he responds saying “Because pirates gotta sing, you know”.
In this episode, Uta told Luffy that She is Red Haired Musician. Shanks told that Pirates and Singing are inseparable.
This surmises the fact that even pirates need to have fun and having a musician in their crew can help lighten up the mood. And since Luffy is all about having fun, he definitely would want a musician in his crew to enjoy the banquet.