They hold the doors to keep them from hurting the occupants of the limo. Those doors are as heavy as major airliner doors. The weight comes from bullet resistant glass and armored panels.
Look at how thick the doors are the next time you see them. I don’t know if they would take a limb off, but they would surely break a limb
There is also a problem with overpreasure when the doors are closed rapidly. This is due to the air tight passenger compartment.
They are also there to slam those doors shut if the principals have to be evacuated.
Doors are opened on both sides when it is anticipated there will be a person or two, even three riding with the President.
The beast isn’t cramped inside, but there isn’t as much room as you would think. The armor panels and glass take over 14 inches off the interior width of the vehicle, and the roof takes off another seven inches of headroom. It is heavilly armored on the undercarriage as well.
It is a very busy vehicle that provides as much protection as an armored truck
Original Question: Why do Secret Service agents always hold the door open on the opposite side as the US president is getting into the car?