Following up on Mac's explanation, consider someone like me who plays a number of different horns. The fingering of the sax family is (almost) the same for each sax, so if I see a note E on the music, I finger it the same way for each sax, whether it is a bari or alto in Eb or a tenor or soprano in Bb. The sound that comes out will be a different pitch, so the person writing the music has to compensate by providing a transposed part. But I don't have to worry about how to finger it.

My trumpets are pitched in Bb (typical trumpet or flugelhorn), C (orchestral uses) and A (piccolo). Again, I don't have to relearn a different fingering system depending on which horn I'm playing (with only a few exceptions for funky notes, such as most trumpets have). I do need to get the music transposed to the correct key (or do the transposition in my head, but that's easier than relearning the fingering I've been using for 50 years).

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