SNAP (Segment Needs A Polish) is a tool for "snapping" a ride to the road it was on. The main purpose is to correct for GPS errors.
Note that SNAP only moves existing points and does not create them. Thus, it is unable to just fill in missing sections.
The problem of automatically correcting GPS data is ill-posed. In the city, where streets are close together, there can be multiple reasonable routes for given data. This site lets the user choose the route for themselves.
It's rarely necessary to correct GPS data since the Strava matching algorithm is pretty loose. But, this method could be used as an additional step during segment creation. That way all new rides are compared to this gold standard.
If you have any more questions please contact me at paulmach -at- raceshape.com. Thank you.