To cause disruption, show how fragile the infrastructure is to asymmetric attacks. Possibly also with an eco activist motivation, to ground hundreds of planes. Possibly also to make an anti-capitalist statement.
I think it's extremely fascinating, and I bet this won't be the last we've seen of this sort of thing happening. The impact is so huge for a relatively small cost and effort.
It depends if five years in prison will be considered a small cost by them. If they do get convicted and serve that long or longer they may not consider it a small cost. Yes the initial cost to launch the attack is small.
On the other hand, the cost of a couple of drones, and only two people in prison, does seem like a rather low cost for the sheer amount of chaos they caused.
100,000+ people kept from traveling, an entire airport basically shut down, military response, the whole lot, by just two people with a drone. That's rather impressive.
If this starts happening frequently, I think the response will become more reasonable. Birds pose a similar risk and the response there is to try to keep them away, and build aircraft to withstand most hits.
I think it's extremely fascinating, and I bet this won't be the last we've seen of this sort of thing happening. The impact is so huge for a relatively small cost and effort.