Concordant Crossroads gives every creature haste for one green mana. That includes your opponents' creatures, making it a card for carefully chosen turns rather than a free permanent advantage. A green deck can use it to activate new mana creatures immediately or attack with an army that has just entered, but opponents receive the same opportunity when their turns arrive.
Elf decks can turn newly cast mana creatures into resources for continuing a sequence. A token deck can follow a large token spell with Crossroads and attack before opponents untap. Those immediate uses help justify the symmetrical effect because you gain something substantial before anyone else benefits. Playing it early with no follow-up may instead make an opponent's creature deployment unexpectedly dangerous.
The world supertype adds a separate restriction: when multiple world permanents are on the battlefield, the world rule normally keeps the one that has been a world permanent for the shortest time. It is not the legend rule, and the restriction applies across players rather than only among your own permanents. In most games, the more practical concern is still the haste granted around the table. Hold Crossroads until the turn when haste materially changes your output, then use the window for a strong attack or mana sequence. The low mana cost makes that delayed deployment easy without spending an entire turn establishing the effect.
