*[Removed previous answer, since it wasn't very clear what I meant.]*

This code is __only__ run when "page wise scrolling" is active __and__ the zoom level <= "page-fit", and __only__ in browsers that support the `DOMMouseScroll` event (only Firefox). The purpose here is the same as when scrolling with the mouse wheel (or similar) in fullscreen mode: to avoid changing page to easily.
For other browsers the scroll event is just handled by the `PDFView.watchScroll` function directly. This means that scrolling will work fine in "page wise mode", the only difference is that at really low zoom levels scrolling *might* become slightly more sensitive.

Since the current implementation in pdf.js uses Firefox specific code for scroll events, I simply reused code for scrolling in fullscreen mode, see https://github.com/mozilla/pdf.js/blob/master/web/viewer.js#L3200

I've tested pdf.js in Chrome, and the mouse wheel scrolling is nowhere near as smooth as in Firefox. Furthermore it is currently completely broken in fullscreen mode.
But in my opinion improving scrolling in other browsers should be handled in another PR (and I don't think I'm the right person for the job).

__So, long story short: if fixing mouse wheel scrolling in all browsers is required for this PR to land, it might be a really, really long time before this gets done ;-)__