What is the meaning of the phrase "for my sins"?

It is an idiomatic expression, For my sins: (British & Australian humorous)

• something that you say in order to make a joke that something you have to do or something that you are is a punishment for being bad.

  • I'm organizing the office Christmas party this year for my sins. I'm an Arsenal supporter for my sins.

Source: Cambridge Idioms Dictionary.

Definitely British/Commonwealth; I have never heard or read the phrase in an American context (negative evidence, I know, but what are you gonna to do?) I have only read it in a British author, never heard it actually said, either. P.E.: ‘Joseph Hoggett, of the Pork Packers, as it happens. Captain, for my sins.’1 Definitely self-deprecatory, often a way of defusing a potentially rancorous situation, and implying that if the speaker hadn't taken some sort of action they wouldn't be in the position they're referring to, as in "no good deed goes unpunished". I agree that the example originally cited is not a common usage, but is possible tongue-in-cheek because it's both literal and figurative.

1 Pratchett, Terry. Unseen Academicals: A Novel of Discworld (p. 470). HarperCollins. Kindle Edition.

For my sins is a self-deprecatory idiom.

But the Op's example is a radical use of the expression.

It would be more typical to say 'For my sins I got two parking tickets in a week'. This would imply that it was one's sins that had caused the two tickets'. Or even better would be the example @Josh61 gives 'for my sins I'm an Arsenal supporter', if Arsenal are going through a bald patch.

But 'going to church'is something one chooses to do. It seems an ironical twist to me.