The only reasons to use one might be if you want to overdrive the bullet past 1600 fps or if you have a rough bore the GC may help reduce leading. If you are using the case full of BP you will probably have a fiber wad under the bullet anyway. Sometimes you can even shoot a bullet made for a GC without a GC with good results. And, these days GCs are expensive. But, don't take my words on it because I have only been shooting 32-20s in revolvers, single shots, lever actions, pump actions and bolt actions for 70 years now.

Agee only two reasons to use gas checks. Velocity in the 1600 fps area or matches that have gas check classes with bullets around 30 caliber

Typical SS match 32 - 38 caliber no reason to gas check. possibly advantage quarter bore but our rules usually prohibit 

One opinion others may disagree 

Boats   

Because, that's the rule.

At current price it ads more than it used to.
.5 cents per bullet is a jump when i cast for plinking and pistol loads  with reclaimed indoor lead that's free.

I use a GC no matter the velocity if the bullet is of that design. No leading at low vel without them, ok accuracy too, but I find accuracy better when the check is in place so that's why I do it. The cost of a GC, even at a nickel apiece, is a small price to pay IMO when best accuracy is the goal. 

I experimented with breech seating .22LR and at the time I only had a couple variations of Lyman/Ideal 225438, no plain based .22 molds. Accuracy without GC's was so-so, but when I defied conventionality and tried the same bullets and loads with GC's in place accuracy snapped right in to a happy level. Ditto my results when working up low velocity loads for a 5.6x35R kiplauf and a Stevens 44 .22WCF. 

My credo: if the bullet asks for a GC, I use it. The bullet knows more than I do!

Sage is so good they are out of stock. 😅