Main benefit, you can detect between trash easier. If you have a good target located between two trashy targets, with a standard coil you may not detect the good target because of the trash being picked up by the larger diameter coil. With the small sniper coil the small diameter allows you get in and around trash to find the good targets. Think of it like painting. You can paint really broad areas with a roller, but to paint the small areas, you need a small brush.
The down side to the sniper coil is you don't get as much depth compared to the larger coils.