Saw a question on Cafe Saxophone about a sax player seeing music with a play along book.  The music for alto sax was in a bit of an awkward key and the music for tenor/soprano sax felt more natural.

Sometimes that’s how it goes.   Might have been written by someone who played tenor or just by a composer that had tenor sax in mind.  Or that might just be how it happens to go.  Classical composers attributed different feelings to different keys.  And there’s probably some truth to that.

And while a particular song might be harder for alto sax to play in the original key, the opposite can also be true, it may feel better for the Eb instrument version. The Charlie Parker Omnibook certainly makes more sense on alto saxophone.  Bird played it on alto in the first place after all.  It even goes out of range (on the low end) for the Bb version so you need to jump octaves at times.

You probably want to learn it in the original key so you can play it with the backing track and with other musicians who will tend to play music in the original key.

But knowing music in multiple keys gives you freedom to play the ideas across many keys with singers and just pieces of it in solos.