Thats because instead of paying the mana cost for spells you cast, you can instead Collect Evidence 10
This tactic is particularly wise if youre playing a colour like blue or green, both of which struggle to remove creatures themselves
If you put an Equipment with job select onto the battlefield with Gilgamesh's ability, the job select ability and the reflexive triggered ability will both trigger, and you'll choose the order in which those abilities go on the stack
The Spiked Corridor side will create 3 Devil (1/1) creature tokens that deal one damage to any target when it dies
I haven't seen Keruga, the Macrosage much outside of Pioneer and Explorer, but it would be interesting to see how people use it as a companion in Commander
You can tap your lands to make Treasure tokens, and that's a very powerful storage of mana until later