'This has always seemed like a weak second move for black to me. Do any of you play this line with black? Can you explain its advantages? Because...'

This is the French Defence, and give white a plus over equal at best (slight better for white, which has plenty of chances of both sides, and with best play, black will still get a draw, even though white may have a slight plus in some factors of the position). It has been used by many greats, none other than Botvinnik, Korchnoi and more. 2.d4 by white is much more common.

3...Nc6?! is probably inferior to 3...c5, followed by f7-f6 at one stage or another.

4.Bb5?! White should take advantage of the fact that black's knight is on c6 (which blocks the advanve of the c pawn c7c5, which is a typical theme in the French) and play 4.d4!

4...Bd7?! Black should not be afraid of the doubled c pawns, as that will actually be more of an asset than a weakness. After 4...a6 5.Bxc6 bxc6 6.d4 c5! is already fine for black.

Thanks for your comments. I agree that queenside doubled pawns can be an asset, and that it is often better to put the challenge to the bishop with a pawn. And I understand your suggestion of 4.d4!, though I was able to play 5.d4! as a consequence of my opponent not challenging my bishop directly.

I didn't mean to imply that it was such a brilliant move that it eventually led to a checkmate, just that the same pawn I opened with happened to end up playing a strong roll in the eventual checkmate from the position it remained in since the 3rd move. Thanks for viewing the game :)