Half-Life: Opposing Force Easter Egg - Why would Valve need to advertise themselves in their game?

In HL: Opposing Force there is a chapter right before you go into the lambda sector where you meet 2 other soldiers. You also find a scientist their and climb some rope. Climb the rope right before the place where the other unit blasts their door to get to you. Look around on the flat part that's connected to the cieling and you should see the logo of Valve all around you.