What it does is allow you to add rifling to a smoothbore barrel.
If firing shot- like Turkey hunting- results will not be good- Rifling tends to spin the shot column, causing a "doughnut" pattern. For shot, use the smoothbore modified choke.
Shooting slugs- specifically sabot slugs- they will do best from a rifled barrel. The rifled choke allows you to use those slugs. PS- rifled slugs do not do well in a rifled barrel. For traditional rifled slugs, use a smoothbore- cylinder of modified USUALLY gives best accuracy.
So, to quit rambling and answer your question- allows you to change between a shot gun and a slug gun fairly quickly and cheaply.
What we have here is... failure- to communicate.