Airplanes in WWI. I know they were pretty new then,

[raccoon]

But how did they contribute to the war effort?
Other than trying to shoot the enemy planes out of the sky, what did they do? Did they shoot men in the trenches?

[Arkansas Granny]

1. I believe I've heard that they were used for surveillance.

Knowing where the enemy is and troop numbers can be very valuable info.

[LuvLoogie]

2. reconnaissance, mainly

[dameatball]

3. They strafed enemy convoys and dropped rather small bombs as well.

The roads sucked for both sides in WWI, so merely knocking out a few trucks and some horse drawn wagons in a muddy area could halt an advance for a few hours.

[Doc_Technical]

4. During the last years of the war,

all the major countries involved had twin engine bombers.
There were some raids on strategic targets and terror raids on civilians.

Russia had a four engine bomber designed by Igor Sikorsky, who
later went on to developed a successful helicopter.

[LSFL]

5. Psychological impact probably outweighed the threat

They added an extra dimension of threat. Even if they were ineffectual, they must have been a distraction for ground forces. One more thing.

[eppur_se_muova]

6. Don't forget that German Zeppelins bombed several cities, including London ....

and the Brits eventually learned how to shoot them down with incendiary bullets fired from airplanes.

As several others have noted, their first real use was reconnaisance. Then one of the pilots fired a gun at an enemy pilot, and things took off after that.

Supply convoys, including trains, were probably more vulnerable to strafing than were trenches. Hitting artillery emplacements before an attempted advance would have been an obvious move as well.