Why do people respond to posts that are several months or even years old? I mean if the OP posted a question, it's highly unlikely that they're still going to be waiting for an answer a year later.
1. A forum post is not all about the original poster. Another purpose of discussion forums is to build a knowlege-base that is searchable by future users. Search engines will direct people here, looking for answers, for years to come. "Late" comments just further add to the base of knowledge.
2. Some people just want to share, or show off, their knowledge of answers. That's good too, if it adds to, or helps organize or explain, previous information in the thread.
3. Some are spammers who are working a strategy. If the comment is nonsensical, too far off topic, or has weird links, flag/report it.
I've been following TFL, either as a lurker or a member, since 2005-ish. Although I don't post nearly as often these days, I still follow TFL faithfully.
There are many OPs from those earlier years who visit TFL these days either sporadically or not at all. People like jmonkey, zolablue, bwraith, proth5, bluedog, tattooed tonka, mebake, txfarmer, ronray, sourdolady, and so many others have made significant contributions to TFL. Thing is, folks who are discovering TFL nowadays have no idea that those contributors, and the experiences that they shared, even existed.
Unless someone resurrects an ancient thread.
The OP may not hang around these days to see a response to one of their posts but people who read TFL today will see that response and then have the opportunity to dig into that OP's posts for more treasures.
It's a good thing.
Sometimes you need the forum and sometimes the forum needs you.
As the newest of newbies, I mostly need the forum. And many, many thanks to all who share their insights, successes and train wrecks.