He simply did not have free space
The head of state came to watch the performance of his spouse-poetess, and, not finding an unoccupied chair, he simply sat down on the steps in the aisle of the audience. However, people gave up their seats to him, but the president modestly refused.
Just sitting on the stairs is quite comfortable. You can always leave if such a desire arises. But this time it did not arise.
Behind the Finnish top officials is the habit of being shy. So, Sauli Niinistö prefers to fly economy class regular flights.
His predecessor, the first woman president, Tarju Halonen, was once discovered while shopping in a discount store.
More information about the features of the Finnish (and Scandinavian) modesty can be found here .