Only you can determine what those pipes are for. They could be part of an irrigation system, or the remnants of some sort of water feature in the area. From the picture that seems to be rocks around the pipe which could be an indicator of a water feature. The caped pipe could be the water feed, and the open pipe from a pump. Is there a pump box or remnants of what could be a pump box in the area?

Have you tried to remove the cap from the one pipe. What happens would be an indication of what it was used for.

Have you identified all of the faucet and valves in your house, garage, or other buildings. Any water feature or irrigation system will have a valve or faucet so it can be shut off.

I doubt the pipe in the picture was used for anything other that water because of its diameter. Is the capped pipe the same size?

There is one exception to that above. We once lived in a house that had the pipes and other remnants of a moonshine still. If you are in an addition, it would be pretty exposed for that kind of activity.