It's here to add counters to things like Dragon's Hoard, Everflowing Chalice and a few other minor things here and there including a land we'll see later
Simply flash a bunch of creatures in prior to your turn, play spells as necessary, hit an opponent with your creatures, untap your lands, and you essentially have free mana to do the same thing again
As for what cards you want to collect to upgrade, Id recommend picking up 5,000 Years in the Future packs
Many Rakdos, Lord of Riots decks just simply have beefy Demons in their deck to ramp out, from Runescar Demon to Ob Nixilis, Unshackled
Even setting the memes aside, having haste and trample on a cheap-ish body holds the door open for Valet to become a future Constructed menace
Keep drawing cards so you always have something to play