All the times, before you sit on a table in online playing portal you are asked " click post to BB" to play fast. why should I click on that button showing post to big blind option?
It is just a rule of poker. When you take a seat you must wait for the BB to come to you before you can start playing. Or you can post a BB to start playing immediately.
Imagine this scenario. A player stands up when the BB gets to them then sits back down when the SB passes. If they don't pay to play then they have avoided paying the binds.
If you sit down in late position then it might be worth paying a BB to play immediately. If you are in early position it is not worth paying a BB to play immediately as you are not only playing out of position but you only have to wait a round or two to come in on the BB.
This to prevent people from gaining an advantage by coming in for free the not playing the blind when it comes to them. Common rule, although not particularly standard in all venues.
They must include this button.
Imagine the scenario, you sit down at a table and someone comes to the door. Without this button the game would force you to pay the BB out of turn. And you could potentially miss a hand.
In order to participate in a round of poker (one full revolution of the dealer button) you must post a BB and a SB. If you skip this process you either must skip the entire round and wait for the BB to come to you, or you must pay the BB you missed to enter into the revolution.
If you sit down at a table you have a choice to either wait for the BB to come to you, or you can "buy into" the current revolution. But since no one can force you to "buy in" until the "BB" you must always be given the option to "buy in" early.