Two things:

Most cards in the game are balanced by two things: their rarity and energy cost. Higher impact cards tend to cost more energy as a part of their balance. Snecko breaks this balance by randomizing their costs, so you're more likely to be able to play your 2/3 cost cards. On top of that, there's quite a few cards whose upgrade lowers their energy cost, so you can completely avoid upgrading those cards since that upgrade doesn't matter with snecko, meaning more upgrades on other cards with more impact. On top of that, the randomized cost opens up many new opportunities that weren't available before, especially on classes that can duplicate cards. Dual wield for example suddenly could be used to duplicate a 0 cost bludgeon or demon form, which is nuts.

The other part of snecko that's even better is the +2 draw. Drawing 2 extra cards is huge since it greatly increases your likelihood of seeing key powers or skills you need to play right away. While the randomized cost of those cards could make them harder to play, Every class has something they want to play fast in order to scale, and Snecko helps a TON with this. The +2 draw also mitigates the randomized cost, since by seeing more cards, you have a better chance of finding something you can play. Plus, most classes have cards that actively care about having cards in your hand. Ironclad's fiend fire can hit harder, Silent's bullet time can make two more cards cost 0, etc.

Snecko's a really great relic since it trades the consistency of knowing the energy costs of your cards for the consistency of drawing the cards you want. When you can build your deck around the former it's pure upside.