Sealing works well if there is not visible mold growth on the item. So like think about sealing an art canvas in a home that had mold in it, but the art canvas was in the living room and the mold colony was in the bathroom under the shower. In that case, any sort of light sealant would trap any possible residual mycotoxins on the canvas so they will no longer float around the air in the home.

But if you have mold growing on unpainted wood joists, for example, because of a plumbing leak, you can't just paint over the mold colony on those joists with a sealant, because it may not penetrate deep enough into the wood, and you can't paint the backside of that joist where it touches the floor / ceiling, and so the mold often will just continue to grow and grow around your sealant. Many paints marketed as mold killers and sealers are totally ineffective at stopping the problem in scenarios like this.