system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does transposing work and why does it exist?

I don't fully understand instruments being transposed. For example, I play Trumpet, which is a B Flat instrument. If I play a C on trumpet, it is a Concert B Flat. So why is it written as a C if it it isn't a C at all? What are the benefits to writing music for an instrument a step up (B flat) or a step and a half down? (E flat)? If that pitch has its equivalent on concert pitch, why aren't all pieces just written in C for all instruments?
gpt
When we say that a instrument is in F#, that means that when you write a C on the music sheet for this instrument and that the player play, you hear a F#.

Transposition is made for 2 reason :

- it's easier for player to read written music : all note will be around the line of the music sheet and you never need to add too much supplementary line above or under that are difficult to reaad (is there 5 or 6 lines? it's difficult to see at first sight)

- it's easier for player to switch between instrument : Alto Saxophone are in Eb and Tenor Sax are in Bb. If you see C-D-E on a part for Tenor sax, you move your finger in a certain pattern. If you see C-D-E on a part for Alto Sax you will move your finger the same way. The final sound will be different ( Bb C D vs Eb F G), but for the player it's easy : same note written => same fingering.