The primary advantage of Blaster Bolts over Bullets is accuracy (which is very important in Sniping).
Bullets are easily affected by wind, gravity and countless other variables. Becoming a professional sniper in real world isn't easy. I remember seeing a documentary about it. Professional snipers need to take even moisture into account before shooting. They need to give lots of thought and uncertain gut feeling and leap of faith are their friends. Also, you should know that success rate of real world sniping is very low when target is moving.
Blaster Bolts simplifiessimplify lots of things because it's much less affected by environmental conditions.
Other than that long range sniper guns (based on bullets) give very high recoil, so snipers need to use very heavy guns fixed with grounds (that's why ahead of planning is required). With this big mobility hurdle, Star Wars snipers can barely work. Remember those bounty hunters who fly on jets, land somewhere and go in position, and change position if target is moving.