Most likely to help NOT casue a collision..I believe what you saw were the strobe lights

I think its just to acknowledge the fact that they are landing and so other aircraft know where they are (for anti-collision purposes).  I know some companies/airlines turn their landing lights on even in the daytime for this very purpose.  Im just guessing...

The answers are correct(re: the reason why it's done....safety/visibility).
The lights themselves are common on certain types of aircraft or as an aftermarket add-on. We have the mod. on our Lear 35. It makes the landing lights and recog. light cycle on and off. I've also seen the same type of system on some of the latter model 737's.
The scenario where this has the greatest advantage is an aircraft descending into a large metro area. The flashing (as opposed to steady burning) landing lights make it easier to figure out which lights are airplanes and which ones are ground lights. In large cities it can often be extremely difficult to even find the airport visually. FS doesn't even come close to doing this visual situation justice!