Marginal cost matters because it defines the competitive landscape. That is, if the market is competitive, price will get driven to marginal cost, because it makes economically rational sense to get there.
For example, if you make pizzas that cost $8 to make, but you normally sell for $20, if someone else starts selling equivalent pizzas at $12, you wil likely be pressured to drop your price to at least $12, or below if you want to beat the competitor. The competitor then will need to drop prices to stay competitive as well. The equilibrium price if the market is competitive will then be at the marginal cost: in this case $8.
Of course, the complaint against this is that it is an unprofitable state. If you're selling at marginal cost, you don't make any money and you don't cover your fixed costs. And, that's true if the discussion ends there. But it does not.
There are a variety of ways to deal with this situation, and the simplest answer is that you seek out areas in the market where things are no longer competitive. One way to do this is to decrease your marginal costs. Thus, if you can now make pizzas for $6 and your competitor still has to make them for $8, you can sell your pizzas at $7.99, still profit, and your competitor can't compete.
Another option is to increase value and differentiate your offering. Perhaps you can invest a little in improving the ambiance of your restaurant, such that the competition is no longer direct. The overall benefit to consumers is such that they're willing to pay a premium.
A third option (related to the second) is to figure out ways to bundle solutions. Sure, the pizza may be priced at marginal cost, but perhaps you can also sell drinks at above marginal cost, and can bundle the two in compelling ways such that you profit on the overall bundle.
The real issue in understanding the marginal cost issue is so that you recognize where there's a true competitive market, and recognizing what will happen to prices in that market, and using that to seek out ancillary markets where you have some sort of competitive advantage and can profit.