This combination places pressure on a number of defensive looks. There is a horizontal and vertical stretch component that makes this sequence of routes so lethal.
To a defense that hasn't strictly prepared for this playcall, the proximity of the routes can cause hesitation in the defensive back's judgement. For example, a "cloud" or rolled up corner back (as in the Cover-2 scheme) can easily get "Hi-Lo'ed" by the flat and curl routes where a hook defender (W, above) can be stretched laterally.
Often times the reads for the quarterback go from 1) corner, to 2) curl, to 3) flat. Although this changes based on defensive scheme and field position.
"Spot" is a fantastic call in the redzone. It floods zone defenses to one side or can utilize a "pick" element against teams using man coverage.
Alternate terminology for Spot includes "Snag."
Outside References
2) Three routes you must stop to win in the NFL
3) An inside look at the Packers' 'Spot Route'
4) Coach Maisonet's Spot Route Concept
5) Simplifying Passing Concepts
6) Brady and Manning All-22: Spot Passing Concept
7) Snag Route: Noel Mazzone
8) Detroit Lions All-22 Breakdown: Pistol Spot Passing Concept
9) Snag and Scat Revisited
10) A look at the 'X Spot'
11) All-22: Snag, Tempo and the Eagles