On normal buses, at some stops, you have 10 people trying to get out the front door, some because it's closest but others because the steps are lower and thus easier and safer to exit. Then you have 20 people boarding... most have their passes in hand, but there are always a few who forget they have to insert their metrocard and end up fishing for it in their purses. Some who don't remember which way the card goes. And some who still pay with change. The longer the bus sits there, the more people get on line to board, and the bus gets stuck at the stop for several minutes. The delay also means more people are waiting at the next stop, and the bus is now overcrowded as well as being delayed.

SBS forces you to pay before you board, and allows you to board through any of the doors. So instead of 20 people trying to get in the front, 15 of them go to the other doors and it really speeds up the boarding. The bus loads quickly and departs on time, which does a better job balancing the load of passengers with the timing of the buses. More or less. The M14A/D has gotten much better since they converted to SBS and made 14th a bus only street, but you still get some overcrowding and delays.