I like to answer older questions, if I have an additional perspective to the question to give.
Adding an additional answer even if there are valid answers can still add value to the collection of answers and questions we are building here. (A late answer does, by it's nature, get not much attention, so it leads to exceptionally low reputation per answer. But hey, that's life, right?) And I feel it's the important thing here: We're answering professional questions in a professional way, and often do that quickly. That's of great value for the general public.
But the real thing of value, that is of value hard to describe in simple terms, is the body of text, the whole collection that we are creating here together. All participants here, whether he or she cares more about asking, answering or collecting questions and answers.
This applies to all StackExchange sites and topics in the same way. In some sites and topics, I like to add questions that have only the purpose of growing the collection, often more academic than practical, and of general interest while not too trivial.
I'm here because I want to take part in the creation of this exceptional body of well structured knowledge.