Composing at or away from an instrument

There are specific benefits to writing away from an instrument vs at the instrument.

At the instrument - Obvious benefit is immediate feedback. This is the same for at the computer. You think, play, hear, repeat... The problem here can be you get set in a certain path, and it can be hard to stray from that path if you want to compose something new. Probably most people will relate with "noodling around" and not really getting anywhere.

Away from the instrument - Benefit here is you tend to focus on different things. You will probably pay more attention to the intervals and the rhythm, vs the sound. This may be good, especially focusing on the rhythm. It is definately a mental exercise and can improve your chops. When you do it, then go back and listen to what you wrote, a lot of times there can be a surprise.

I would suggest doing both occasionally. One of the main benefits of composing away from the piano is increasing the mental work it takes to compose. This will change your brain, and open up new paths.