Its an easy way around building a properly set up distributor. Lots of people will just lock it at peak timing so as not to have to play with the advance curves. Very few people have the knowledge and equipment to set up a proper custom curve these days. I guess thats why my old distributor bench is kept pretty busy. I learned a looooooong time ago how to do em right.

I much prefer a well set up unit as it is much easier on the starter motors, allows for a decently low initial timing setting to get idle speed down yet still allows for whatever total timing and advance rate the engine wants.