What do i need a trommel for?

[DaveAllOver]

the sorter already separates the ore from the pay dirt... what do i need a trommel for? does it do something special?

[arcturus]

It does not do anything special as such, no. However, if you place it after your washplant, you can separate the dirt off and direct it to a sellpoint/different container, leaving you with just "valuables" from the top output (i.e. less clutter in your containers).
If you put a trommel in after a fractioner, you can divert the gravel out from the lower output to produce cement or asfalt, while the ore-fraction from bedrock will be going out of the top output on the trommel.
So I guess this was a fairly longwinded way of saying, it is mostly a convenience thing for sorting your ores
Hope it helped and happy digging. /ARC

[Be curious, not judgmental]

https://www.youtube.com/watch?v=9YbNf4WNPKk this may help you better