valve connection benefits flgxweld vs flgxflg

Just a guess.

Let's say you have a butt-weld system where all valves and most equipment connections are butt-welded. But, you want a place for a "Spec Blind" for system isolation. This would be a good place to install a Butt-weld by Flanged valve, the Spec-Blind and the mating Flange.

It is a good question but why do you ask it?
Have you seen this somewhere? In the field or on a P&ID?

I thought of another one.

We may have a special Reactor vessel in a very high temperature, high pressure system. Due to the nature of the process the Reactor is "Cladded"(weld overlay or explosive applied) or glass lined. all connections must be flanged and no welding is allowed. The piping system will be all welded. So the first valve against the vessel nozzle must be flanged on one end and welded on the other end.