First is testing the response times, locations, scale, etc... If you intentionally violate air space, you can measure how long it takes for your future opponent to scramble their own response. You can see what kind of equipment they send to respond. You can see where the response comes from. You can see how many combatants they send to respond. That is all valuable info if you’re planning potential attacks in the future.
Second is creating a “boy who cried wolf” situation. Repeatedly violate their airspace, and they’ll begin to see the violations as routine. They’ll stop treating them as genuine threats, and will simply start treating them as just another annoyance. None of the violations ever result in an attack, so why would this one be any different? Then when the actual attack comes, your opponent is caught with their pants down because they weren’t fully responsive.
Third is money. In the grand scheme of things, flying a single drone around is a drop in the bucket. In contrast, a sudden scrambled response costs a lot. It’s a lot of sudden manpower and logistics to coordinate. Especially when the violation turns out to be a nothing-burger. If you want to begin straining your opponent before a war even begins, make them start counting pennies before you attack.
Fourth, it could be posturing. Look at how weak our enemy is! They can’t even patrol their own border. They’re afraid of us and let us into their airspace. Etc…
Fifth could be intimidation. Remind them that they’re within poking distance. Especially since they’de supporting Ukraine. Want to discourage that support? Encourage them to circle the wagons and bring their support home instead.