As a card itself, it's so powerful it's one of MTG's famed Power Nine, nine insanely strong cards that were only printed in the very earliest sets (long before the game blew up).

So it's not only so strong as to have almost always been restricted or banned from every tournament, but it's super rare due to the game having been so niche at the time (and in the rarity is its value).

As for Black Lotus' actual power, it costs nothing to play and adds three mana (the resource with which you do basically everything, from casting spells to playing creatures) - three mana is a huge advantage (you gain mana from land, you can only play one land per turn if you have one in your hand), as you can play this on any turn to gain a huge amount of tempo; it lets you play big guys when your opponent only has the mana to play little guys, it lets you have the mana to flood the board with creatures when your opponent doesn't have the mana to deal with it, it lets you play a big removal spell to get rid of their best card while you still have all your other mana to do other stuff with.