Figuring out the best time to do it depends on your deck, what your opponent might do, and what you want to achieve
Pyroclasm, for instance, reentered Standard in Duskmourn earlier this year, and while regular copies go for mere cents, textless Pyroclasm is now $16, up from $3
You may choose an attack from a Pokemon you find there and use it as this attack
Its ability lets you search for an Stage 1 and evolve it if its nearby another Eevee evolution
Sharing more than one color or more than one creature type with the equipped creature does nothing
This gives you less reason to keep the Plains because you want a density of threats to fight through a slower deck, and the game is also more likely to get to a point where you naturally draw seven lands over time