What can weir be used for?

"Weir" can refer to several things. To give you a helpful answer, please clarify what you mean by "weir."

Do you mean:

* A weir in a river or stream? This is a structure that controls water flow and measures its volume.

* A weir in a swimming pool? This is a type of overflow drain.

* Something else entirely?

Once I know what you're referring to, I can tell you what it's used for.