The knee pad the soldiers wore, is there anything special about them ? Also why were they wearing them ? Is it becuase they came in from the air ? Becuause it was an urban environment ? Or do Rangers and Delta always wear knee pads ?

In field, and combat environments, one has to often lie prone.
Sometimes you have to drop rather quickly, and often into the prone position.
This can cause bruises and scrapes to the knees and elbows.  Over time, these minor injuries can cause discomfort, and even an unconscious moment's hesitation when dropping to prone.

Like wearing a helmet when you snowboard.
It makes you feel safer, and go faster.

Knee pads, as understand are issued to anyone in a combat role these day. Something to do with injuries to the knees in stressfull situation. Of course I could be wrong.