mtg lifegain commander deck Liesa, Shroud of Dusk LIFE DRAIN Tax Magic Custom How to Build a Lifegain

A few classic pairings explain why these matter

After Ether's ability resolves, the next instant or sorcery spell you cast that turn will be copied whether or not it has targets and whether or not you spent the mana added by Ether's ability on that spell

This suggests that the deck will be stocked with hero cards, and it will be supported by either equipment to further increase the damage output of the creatures, or instants and sorceries to help maintain a strong board presence while keeping your opponents' boards light

It gives the world texture

For a very solid and highly detailed look at this aspect of MTG collecting, I recommend this fantastic article by Guo Heng Chin