Why is this move considered bad?

Question

I'm trying to play against simple online bot cosmi.net. I started playing about two days ago, so I'm a complete beginner, and even the easiest version on 9x9 board is really hard for me to beat. This is in fact the first time I managed to do that.

$$Wcm12
$$ ---------------------
$$ | . . . . . . . . . |
$$ | . . . . 4 3 . . . |
$$ | . . . . X O . . . |
$$ | . . . X . O . . . |
$$ | . . . . X O . . . |
$$ | . . . X . X O . . |
$$ | . . 1 2 . X O . . |
$$ | . . a . . 6 5 . . |
$$ | . . . . . . . . . |
$$ ---------------------

For some reason though, the bot marked black 17 as a bad move, and suggested to play at point "a" instead. Can anyone explain me why was that move marked as bad?

Answer 1

It seems that the AI thinks the move is bad because White can live in the corner. I don't see a way to do it, though. I think your move is okay. It's strange that the bot marks the move as bad, but didn't try to live during the actual game.

Added:

I don't think that works, @jknappen, but it may still explain the computer's advice. After this sequence, which prevents the ko:

$$Wcm12
$$ ---------------------
$$ | . . . . . . . . . |
$$ | . . . . X O . . . |
$$ | . . . . X O . . . |
$$ | . . 8 X . O . . . |
$$ | . 6 7 . X O . . . |
$$ | . 4 5 X . X O . . |
$$ | . b O X . X O . . |
$$ | . 3 . 1 2 X O . . |
$$ | . . a . . . . . . |
$$ ---------------------

White can force at E6, and then has a nice endgame sequence starting at E9. So it's better to play E2 originally.

Answer 2

When White "tenukied" with 16, you had a chance to kill 12, the stray white stone in your territory. The killing move is "a," which works well with your nearby Black stones. He will make points by extending from 16, but that is the compensation you will have to give for killing 12 with a.

Your 17 answered White directly, giving him the best of both move 16 and move 12, because he can now save the latter stone by playing "a" himself. Now he can save the two stones by either jumping to b5, or by connecting to 16. These two moves are "miai."