It is good for AY because it undercuts their own website.

OTA uses this "undercutting with their own money" only when market demands it.
Tickets sold thorugh the aggregators are aimed at buyers looking for lowest price. Most tickets sold there would not be sold unless you offer lowest price. OTA and aggregators allows Finnair to keep a relatively high price at their own page, and still attract the cheapest crowd via the aggregators.

I think elventhor has it right. It is about ancillary revenue. Someone decided increasing ancillary revenue is a more important target than all others. Thus it is more important to only sell tickets at their own web site, even if that means lower number of tickets sold.