There are many markets. We assume you will be interested in selling on all our websites (= site markets). There are currently 13 of those, but of course this can change, this is why there is an API method to retrieve all available markets at any given time → /refb.merchant.v1.MarketService/ListMarkets / ListMarkets().
If you choose to not use MarketOffers, all prices for individual markets would be taken from the offer reference price. So you lose the ability to set individual prices per market and setting only one price per offer for all markets. Due to different price structures in different regions you may not be able to compete for the buybox which is very likely not desired.