and to further expand on the concept....since btn has position through flop turn and river the BTN should be playing a wider (weaker) range of hands since they can react to the actions of the out of position players, so the blinds can defend their blinds to a BTN steal since their range is typically stronger than the stealers range. for this they need fold equity so their resteal numbers should be reasonable. if someone is 3betting every time the button steals , btn can assume that they are likely 3betting light and then 4bet themselves lighter than normal(otherwise known as levelling wars)
.