Acts as a preservative. I don't add hops, because I like doing starters in a clear bottle so I can see the yeast layer, and I don't want to have to worry about skunking.
I've seen both opinions, with no definitive references. They do have antibacterial properties, that is known. Are they necessary? Again, I don't know. I use a couple pellets. Just paranoid, I guess. They don't interfere with being able to see what's going on.
Acts as a preservative. I don't add hops, because I like doing starters in a clear bottle so I can see the yeast layer, and I don't want to have to worry about skunking.