builders. bridge/saddle question

I am close to making the bridge and saddle for my lap steel project. it is an acoustic /electric.
it will have a cedar top over a hollowed mahogany body. it will look more like an acoustic guitar than a electric.
I am going to use an ebony blank and bone saddle.
I have been looking at many bridges and have seen angled and non angled saddles.
why is that?
I assume string gauge changes low to high factor in. but why the varieties.?