They represent one of the different elements used in the game: Fire Water Grass Lightning Fighting Psychic Metal Darkness Fairy Each basic energy card provides exactly one energy of the relevant element
It's "just" a one-for-one, but if you either drew more Lands than your opponent or they just had a light draw, it could result in some un-fun non-games, especially in an era with less friendly mulligan rules
Conversely, lets look at Kassandra
Its good in specifically gnome and artifact-heavy decks as a source of creatures
often, youre going to have a turn two mana rock (like a Dimir Signet or Planar Atlas), which youll look to follow up setting up with a Nazgl or engine piece like Archmage Emeritus or Talrand
If the $55 price tag for near-mint (NM) copies is too steep for your budget, then lightly played (LP) copies might be more appealing, ranging from $42 to $49