When a new player sits down, he or she must either post the big blind or wait for the big blind to come around again. This prevents players from gaining an unfair advantage by getting dealt free hands.
Instead of coming in the middle of a rotation, some live poker rooms don’t allow a new player be dealt in until it is his turn to put in the blind.
Waiting for the big blind to come around rather than posting right away has two main benefits. The obvious one is that you will not have to post two big blinds in an orbit. You also get the opportunity to see how all the other players are playing and try and get a read on them before you even sit down and start playing.
All online poker sites give players the option to select the “Wait for Big Blind” checkbox, which if checked will not post any blinds until it is your turn to post the big blind.