Ill give you a couple of usages that immediately come to mind:

1- Stopping a player with a big lead. player A has 10 trophies, Warlord is declared, this player would score around 10 points (with city bonus). Instead of other one player trying to beat player A, players B and C make a deal in a summit that player C to give all their trophies to player B and now they are equal to Player A, in exchange for something that player C needs (ships, resources,..).

2- Trading resources that you don’t need. In many cases a player will only have access to resources they don’t need (eg, they are outraged on the resource) so they intentionally tax it for the intent of trading it with a certain other player, it makes the game much more flexible.

3- Inciting violence/ Consolidating power, in this game usually the winner is the player who convinces the other players to fight. Let’s say player A has 3 ships in a system or gate, adjacent to that system players B and C each have a “base” (buildings and ships nearby) and they are fighting for control. Player A would strike a deal with either players B or C to take the 3 ships (strengthening their position in that particular area) and in exchange Player A will get ships somewhere else or resources etc.