At any point of any poker tournament, you'll hear players refer to their stacks in terms of big blinds. "I started the hand with 27 big blinds" or "he had effectively 15 big blinds" are commonplace in any verbal hand history these days, and rightfully so. In tournament poker, stack sizes are extremely important.
While a player's stack size is extremely important, it's also something that changes frequently, especially when antes kick in and you're putting chips into the pot each hand before cards are even dealt. It's no secret that there are more specific plays one should make based on a stack size in preflop action, especially for the more beginner or intermediate player.