A gas check has several benefits, depending on what you are trying to do.
First and more common reason to gas check is to drive a already hard bullet faster without leading. The gas check kinda scrapes off freshly deposited lead. The gas check also grips the rifling to prevent skidding.
It will also allow you to drive a soft cast bullet at normal hard cast plain base velocity. It will still clean the barrel and grip the rifling, those benefits are always there.
The other benefit of a gas checked design is that it will let you cleanly shoot slightly undersized bullets. On the chrono I'm seeing some velocity spreads as the bullets are not sealing uniformly because of the size variables, but they are still fairly accurate, and don't lead at lower to mid velocities. And undersized plain base bullet will lead in most instances, add a gas check and it probably won't.
IMHO, a gas check is a gift to amateur casters, a crutch to obtaining immediate success with less than ideal bullets.
The casters that can shoot plain based bullets at high velocity with good accuracy and no leading have all my respect, that can be like searching for the Holy Grail for a podunk caster.
I've run some pretty ugly 358156 bullets in oversized bores with decent results. I also just ran some C429-240-SWC LEE at 1000fps with hardly any leading, and my alloy was only about 14 BHN.
My pet peeve: LEE molds for the 44 cal pistols are designed to drop 429 bullets, which is the minimum spec size for a 44 Mag bullet. Saami max bullet for 44 Mag bullets is 432. I've asked the arrogant tech support persons at LEE why, but all I got was a lot of attitude.