One arm wall HS

Something I've been doing lately is get to a HS against the wall and lift one hand. Then keeping the one arm HS (feet still against the wall) for 30secs.

Is that a helpful exercise for anything?

Not really.

It would be better to master your freestanding handstand...

And from what I've found it's better to work one arms away from the wall by leaning and keeping good HS form.

Conditioning work such as wall runs/hs shoulder taps can be done for similar aspects, and once done well on the wall can be progressed to freestanding as well

Well I have a 1min+ freestanding HS so I meant it to be a good exercise for strength of forearm and deltoid, not as a progression to one arm freestanding HS. I feel one arm freestanding HS is more of a balance thing.

I spent a bit of time doing as you did, one arm HS against the wall. While I don't necessarily agree with brain that it's not really useful I think you could get away from the wall. I found that with my feet against the wall that it really reduces the strength required to hold the one arm HS but also the body angle just isn't right thus, possibly, promoting poor muscle alignment training.

Since you're fairly far along with HS I would suggest instead that you find a prop, bucket, something elevated 12-20 inches on which to place your free hand. When I'm practicing this I enter the HS with one hand on the ground and the other elevated in this fashion then gradually attempt to shift onto the loaded arm practicing the body alignment necessary. (You cannot really do that well against the wall...)