It is mandatory to understand that some survivors are better at taking out certain zombies than others. Check the strengths and weaknesses of each survivor to know how to use them more efficiently.
There is no point in letting a sniper defend against a horde of small zombies. A survivor with a flamethrower or minigun will do a much better job.
However, the sniper can take out spitting zombies from much further away, especially when he is in a tower. Snipers are also great in killing large zombies that you shouldn't even let close to your buildings. They will smash them in seconds. Keep them at distance!