It’s quite important to join Student Union overall.
It’s different from Japan or Taiwan where every student would receive his/her own student card from the university naturally, but in Finland if you don’t pay to join Student Union, you won’t get your student card.
More precisely speaking, you need to pay around 100 euros every year in order to get a sticker to prove that you are in the Student Union. You stick the sticker on your student card to use it.