A (naive [in details] weak) example i regularly use is electricity:
Imagine you own solar roof panels and produce more electricity than you need. You want to sell it back. But you want to sell it at the highest possible price to the best payer.
The centralized concept is that you sell it to your main provider and hope they pay the best price (or that your are able to move cheaply/quickly enough). Or that you sell it to a intermediary (or price-comparison service) that hopefully has the lowest rate (or that your are able to move cheaply/quickly enough).
The decentralized approach to this is to announce the sale on the network/market and pick the best offer.