Resources in this game remind me of the Yu-Gi-Oh! TCG.
Mana=cards with powerful and versatile effects with little risk but powerful costs to make up for this. Mana's a powerful resource due to how you can use it, because the only downside to mana is running out of it. Due to their power and versatility, mana-using Champions are needed -- a.k.a. they're "Staples". It's highly unlikely you'll ever be able to win a competitive match without a mana-using Champion, because without them your team becomes telegraphed and easily manipulable. They're balanced to have mana because without the need for that resource they would be completely broken; if they run out of mana, they simply can't perform, so they have to retreat and regenerate.
Energy/Manaless/other resource=cards with very powerful effects with little-to-no cost that can only be activated by doing a certain thing, like attacking the opponent or destroying a monster. These kinds of cards -- and Champions -- aren't considered Staples by any means, but they can get you out of situations that would otherwise put you at a huge disadvantage. These Champs are balanced by having their abilities based on certain actions (like attacking) because if they were to have those abilities balanced around using mana they would end up being far too powerful for what they do. Because they don't have mana, they only need to worry about having their health depleted -- therefore they can fight longer than mana-based Champs can, which puts them at an advantage when done correctly.
Like in Yu-Gi-Oh!, the objective in League of Legends isn't purely to win the game...it's to use your resources in such a way that your opponent can't make a comeback. The less likely they are to make a comeback, the less you have to worry about variables...which, in turn, lets you focus more on winning. Energy and Mana each have their downsides, but if you can play around these downsides and use their upsides effectively you will come out on top.
