Not all writers work that way, but a lot need to get the whole thing written as a solid chunk to get an overall view and so see what needs to be changed.
And then revised.
From the top down. (You don't want to worry about grammar in a passage that you are going to jettison.)
But "don't get it right, get it revised" is a rule that needs to be reminded of, often. Hack out the scene and smooth out the edges, so they rejoin, but don't spend forever on it. There's always the next pass.
