Forgive my enactment, but it seems like the easiest way to explain: Let's say, hypothetically, it's a piano piece. I'm on a train, composing on paper, no piano in sight. I have a strong idea for a part in the bass clef. It's fairly simple rhythmically, spanning about 10 measures, but the notes vary in each measure. I then start working on the treble clef. I had a vague idea to start, and it evolves as I write. I want a complicated run in measure 5. Ooops! I don't have enough space to properly coordinate with the bass clef. Time for the eraser. Or, I re-write bar 5 on a different page and worry about it later when finalizing. Or, maybe next time, I space out the bass clef part extra-wide, just in case I decide to cram a bunch of notes above it. Ugh!
I assume this is a problem solved by others long ago (centuries ago), and there's probably a set of best practices somewhere. However, I can't find them. Notation software obviates the issue (obviously), but I'd really like to get more comfortable notating by hand, well.