Hey,
This is not about reading a single line ok, all non-polyphonic instruments never deal with this, my concern is in organ pieces mostly, and other might looks similar.
We pianists, honestly, always have more time to study before the performance, no matter how impossible is the score, we have time to study, this is not the case in organists.
Take a look to the following score, let's say you have to play it as perfect as you can on your first attempt, try to determinate how exactly do/would you read it, what do you see first, how much do you use your harmony knowledge to guess the following note, where there notes you didn't see, only guessed ?
how do you read it, up to down on each quaver ?, look a particular note, the others like half sight ? sight focuses on every single note ?
In Orchestral scores, I can read way more lines, but the fact that I don't have to play it, only to know what they should do, makes it different.