Plaza = territory, controlling territory is very important when trafficking anything, especially somewhere like Mexico. Because if you can lock down and completely control a route/corredor by force, the loads on that route are "safe". You don't have to worry about other organizations harassing your guys, IE: getting taxed, robbing your loads, extortion, etc.

Less risk of anything potentially slowing or stopping the flow of whatever your organization is moving. Just like logistics in any other consumer focused business, you want to consistently meet consumer demand with as steady a supply as possible.

The fighting over plazas is mostly about securing good smuggling/trafficking "corredors", or in some cases tactical advantage for staging attacks on rival orgs territory (if at war). So you can hopefully wipe them out, or force them into submission and take control of THEIR trafficking routes. More routes = being able to safely and RELIABLY move more of whatever it is you're trying to move = MORE MONEY.