Why does this community helps people for free? How can we return the help to Arduino community?
nityoday:
Why does this community helps people for free?
That seems a very strange question, and a strange Title 
How can we return the help to Arduino community?
The simplest way is to read Forum Posts and give advice to people with questions.
...R
nityoday:
Why does this community helps people for free?
I enjoy helping people and sharing knowledge.
nityoday:
How can we return the help to Arduino community?
Participate by trying to help others with your knowledge. You might encounter a question/problem that you once had and now know the answer to.
Every time there is an "interesting" question, I learn more myself in the process of answering it, and by answering here on the forums, I potentially help more than just whoever asked the questions.
I also learn from the answers that other people post, and some questions and/or projects are thought-provoking even if I don't have an answer. Heated arguments between actual experts are especially educational, though occasionally unpleasant.
The more common quick answers to easy questions are essentially my "payment" for the questions and answers that I learn from, and in some sense my "donation" (in lieu of cash) to the Arduino "community", for all that non-arduino hardware I've bought or made.
I've looked at forums run by vendors, where customers ask questions that they expect to be answered by employees of the company. They tend to suck. 
westfw:
Every time there is an "interesting" question, I learn more myself in the process of answering it, and by answering here on the forums, I potentially help more than just whoever asked the questions.
+1
sterretje:
I enjoy helping people and sharing knowledge.
Participate by trying to help others with your knowledge. You might encounter a question/problem that you once had and now know the answer to.
+1