Letters in animal crossing seem like such a waste of time, but I find myself sending them to townies every so often. Of course, since they can't read, I like to send them harassment letters and other novelties such as:

Dear Rosie,
thank you for that nice shirt. It looked even better when it was all wet on my hard body.

Dear Frita,
Stay away from my flowers. I will fucking kill you. Seriously. Fuck off.

Dear Bill,
You're a cool guy...but don't take this the wrong way, I'm just not into you. Sorry.

Dear Frita,
Can you move out. You're scaring the pretty villagers.

Dear Frita,
Thank you for the picture of you. I'm coming to kill you tonight.

Dear Vesta,
Say flaffo one more time and I will stab you in the belly and spill your intestines.

I think it's just hilarious how they come up to you and are so excited about your letter, and are like 'thank you for your letter! I read it every night'.

Why do they even implement such a feature if all the answers to the letters are pre-written and don't consider what you're saying at all?

I guess it's just another thing about Animal Crossing that I just don't and won't understand. I understand for little kids though.

I used to quite a lot. I always make sure to send them random junk with the letters though, because then they send you stuff back, and you might get lucky.

Well, I can't expect a game to be able to comprehend anything that's being said. Not even huge AAA pc titles have that kind of functionality.
But yeah, why is it there in the first place is a good question.

Well softwares like Cleverbot or Alice or even Siri have done it so I don't see why a game like Animal Crossing that sells millions of copies can't afford to make an AI system (at least a simple one). That oft results in even funnier outcomes, especially if the AI is simple.

thats true. we can hope for nexf time.
I do like siris silliness.

I could be wrong, but aren't certain word combinations, like "Get well soon!" or "Do your best" recognized? If not that, maybe it was that certain words might bring out a happier response. I sent a sick villager that sort of letter one time, and they seemed to like it more than usual.

I think my favorite thing about those letters is having a resident from a friend's town show you you one of their's. I've seen some pretty priceless ones.

It's to boost your friendship obviously! Also if you send them an item, they'll send you something back. If you're into collecting absolutely everything, then you can also get different types of letter backgrounds this way. Lastly, your neighbors will show off your letters to anyone.