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 simplifies lots of things because it's much less affected by environmental conditions.