It can't attack unless you've controlled it continuously since your turn began, it can block if it's untapped, it can be tapped to pay another Vehicle's crew cost, and so on
For example, many Avatar cards get stronger as you cast lesson spells, like Katara, Seeking Revenge or Sokka, Bold Boomeranger, and other cards care about having lesson cards in your graveyard
Over time, this sort of hoarding can move the price of a card significantly
Lrien Revealed allows you to draw three cards at sorcery-speed for five mana, making it the most expensive card in the deck and a poor mana investment unless the game has gotten really drawn-out
We've got a separate article that explains the full history, and how it came to light