Yes, they do shield against accidental knocks and prevent flaring. I just bought a better hood (The one made for EFS 17-55) for my 17-40L on wed. Even though it might not have an immediate impact on my photographs, I know it'll help in the long run (as long as I stick to 1.6crop cams haha).
Since you have it, might as well use it-- It's meant to be used, y'know =)
Shields your lens from stray light hitting the lens from other angles and causing unnecesary flares. Does not have to be sunlight. Also can be a form of mechanical protection against accidental bumps to your lens.
But to what extent it improves your image in all other instances is debatable and you are to judge yourself though. Else just use a makeshift cap ( I do that sometimes when overseas travelling ) Personally I do not use them because it adds too much bulk to my travelling gear.
And it is not like you can share a lens hood Apart from the obvious differences in diameter of lenses that you might carry, using a non customised lens hood for a particular focal length may cause mechanical vignetting to occur