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In a regular cash game, you need better poker hands to call a raise or to reraise than if you raise as the first person into the pot. This makes sense: someone has already announced (by raising) that they like their hand, so you're going to need more of a hand to get involved. In a tournament, regardless of what game and what betting structure you're playing, this effect is magnified. It's called the “gap concept,” and it was first described by David Sklansky, who most people agree is the top poker author.
In a tournament, you want to open-raising more pots than you would in a cash game, and defending your blind less. The reason is that there is value in a tournament to just staying alive, so you don't want to call raises. And everyone else realizes there's value to staying alive, so they don't want to call raises, so you should steal a lot.