Selling off a share(even a small one) of a business is to raise money. With the huge success of Stellaris, and HOI4 just about to inject much cash into Paradox, why was now a good time to seek investors, when presumably, the cash flow situation should be very positive.
Expansion. That's why. In fact, with the success of Stellaris and the upcoming HOI4; the share prices should be high. So another massive cash inflow that can be invested into even bigger new games; or DLC and support for existing ones
They (the owners, not the company) are selling already existing shares, therefore the company will not see any new cash coming in. It could open up some strategic benefits though to have new owners, if they open up new markets (geographical / platform in this case) through their current portfolio and connections.
It could also help valuing the company if shares are traded publicly, so it is easier to purchase other companies for strategic mergers, using non cash issue (shares) to pay for the deals.