If you keep all of the low candy cost evolution Pokemon (such as pidgey or weedle) to evolve, you might be keeping more in your box than you need. For example, if you have 178 pidgey candy and 68 pidgey, you could evolve 16 of them, but what you want to do is transfer out 45 of those pidgey, which would allow you to evolve 20 pidgey, without transferring out too many such that you would have candy for another evolution, but no pidgey to use it on.