Yeah, using OpenVPN allows your client and server to reside on the same subnet, which I presume is necessary for Steam In-Home Streaming.

You need the openVPN for the Steam IHS Lan discovery and upd streaming. With my setup I still use Hamachi which also does the job and was much easier to configure. Downside is that it is proprietary and has phases with extreme paket loss which make playing impossible every now and then.

For this setup, yes - a VPN is necessary to use Steam's inbuilt streaming feature.

The idea is OpenVPN enables IP multicast to be tunneled over the public Interent, which Steam's streaming requires.

you're not playing over rdp; that would be terrible.

you're using steam in-home streaming through the vpn.