For me fly tying cement has two main purposes. One is to hold the fly together and the second is to coat a pretty head on the flies. I make my own head cement so since I want the flies I tie to be durable I use a vehicle which enhances its ability to penetrate deeply into the materials and thread. This produces a fly which is almost indestructible. In order to coat a pretty head on my streamers I use my thicker head cement.