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If this is one of the revolving doors I'm thinking about (in CDG 2A IIRC), it's due to the fact that, contrary to many airports, arriving and departing flows are on the same level, while they need to be kept separate.
There's a corridor along the length of the terminal (on the side where the planes are) that connects all gates to immigration. But this corridor needs to be crossed by departing passengers from the departure lounges to the gates.
I believe there's one such revolving door for each gate or something similar. If the gate is not used, the door is in a position where it lets everybody in the arrivals corridor go through (that's probably the reason you think there was only one such door, while I believe there are many more). As soon as there are departures on a gate, it goes into revolving mode, alternating between arrivals-to-immigration and departure lounge-to-aircraft.
The doors are actually visible on the CDG Terminal 2A map, though the map is not really that precise (it doesn't show the full plane-to-immigration-to-baggage reclaim path). Here's part of the map, with the crossing flows (arriving in red, departing in blue) added:
As pointed out by Zach Lipton in the comments to the OP, the door is a Flowside door, and here is an illustration of the flow separation:
Keeping crossing flows on the same level was obviously not the greatest idea, and it was replaced in more recent CDG terminals by separating the arrivals and departures flows on separate floors.
2C has the jetway bridge "moving" ("swinging", really) between the top (departures) and the bottom (arrivals) floors. 2F has double jetway bridges, one for each level (very visible from the outside). 2E L and M gates have elevators/escalators sending arriving passengers to the top floor. Don't quite remember the setup in 2B, 2D and 2E K gates, though 2E K gates definitely has separate floors, and 2B/2D are planned to be retrofitted with a second floor to separate the flows, so it's possible they have the same setup as 2A.