Whereas in Word your manuscript is the file, just one big stream of text, in Scrivener you can keep everything in nice tidy chunks. Why's that important? Well, first and foremost is speed.
You can zip from one chapter to another in seconds, and since each is titled with a name you provide, making a quick jump to review what happened in a previous chapter is just a single click away, rather than minutes of scrolling around in Word trying to find the right spot.
I love Scrivener.
There's a reason the app has so many fans: it makes writing easier.