I suspect it is reconnaissance to use IF a plane gets shot down. The response would be to hit a NATO target to show a limited conventional response. Or, if a NATO country assists Ukraine more directly. an attack would happen.
If NATO were to directly shoot down a Russian plane over Ukraine (as opposed to enable the Ukrainians to do it), then Russia might conceivably retaliate against a NATO target. However, if a Russian plane were to be shot down over NATO territory, history shows that they would bluster but not make a bad situation worse. By history, I'm thinking of the incident over Turkey in 2015 described in the link below.

