It shows a theatre - or someone - took notice of your play, and they took time to present it in a reading in front of an audience. As far as how to list them, I think Readings works. If one has been "staged" you can use (parentheses) next to the title and put "stage reading."
Or, if you do the reading yourself - as you recently inquired about funding in a posting - just mention the space in which it was presented.
I would include it. In addition to what has been said above, it also shows that a theater respects your opinion, knowledge of the theater and are able to provide reliable comments.
Some plays I read are a definite "yes", most are a definite "no". I give those in the middle to a few trusted friends to read. I trust their opinion. Seeing that on your resume would tell me something positive about you.
FYI: When I circulated the TORSO script, one opinion was "I don't like it but I think you should do it. It's cutting, edgy, outside our usual fare and I think the audiences would love it."