Comment by skybrian

Sure, a sufficiently smart compiler can do what it wants, but it's often conceptually a pipeline and could be implemented as one in debug mode, without having to rewrite the code. Not in production, though, since you don't want to store stuff in temporary files when you're not debugging them.

In some languages, a series of assignments and a large expression will often compile to the same thing, but if written as assignments, it will make it easier to set breakpoints.