I don't quite understand why someone would go to the lengths of apostasy.
1) to decrease the political power of the church in Poland (which is based on church's constant talking about the statistics)
2) sometimes it's necessary to leave the catholic church when you convert to some other religion
3) because you don't want to be a godfather/godmother or a witness at a wedding and you know the only way to oppose the pressure of your family and village is to be formally unable to perform that function
4) you don't want your family to play you a trick and make you a catholic funeral against your will - if you're an apostate, they can't do it
5) you think catholic church is an organization that is harmful to your society, therefore you don't want to be a member of such organization
6) you hate the fact that you formally belong to some organization and nobody has ever asked you if you wanted to join it
There are many reasons. Anyway, it's just about writing a paper, giving it to your parish priest and visiting your parish from time to time to remind the priests that they have some formality to do in case they 'forgot'