I like the trading aspect and being able to trade future resources.
I started a huge galaxy game with nothing but advanced start AIs (normal difficulty), and one of my neighbors jumped on me early when all my minerals had been going toward stations and colony ships and planetary upgrades before I had a chance to build up a decent fleet. I had no minerals, but my mineral income was great.
So I traded a chunk of my mineral income for the next 30 years to every friendly empire I knew in exchange for all the current minerals they would part with. I traded up several thousand minerals and got all my spaceports churning out ships. I lost one spaceport in the buildup time, then completed a fleet equal to his (and over my limit) and hammered his with a swarm of missile corvettes. I knew he was rocking nothing but plasma with no PD after the loss of the spaceport, so I was confident sending in my missile armed, armorless, shield heavy corvettes. Due to plasma not being so great vs shielded corvettes, they wiped the floor with his fleet, losing only a third of my fleet power. I then went through and blew up all his spaceports. Now the big neighbor who tried to jump on me is paying me 25% of his income, more than I traded away - and after losses I was left with a fleet just barely under my limit to protect from my other unfriendly neighbors.
I also tend to trade away minerals for energy after I've built stations on every rock in my territory and end up with negative energy income and hundreds of mineral income. A lot of the AI seem to enjoy trading future energy for minerals 1:1 and I guess it's the way I play but I'm always short on energy income. But if I trade away 30 minerals and get 30 energy to power the 30 stations on 2 mineral rocks that otherwise wouldn't have been built I'm still gaining 30 minerals a month.