Hi there.
I need your kindness to help me forming a question:
Is this question correct :
What are road signs for ?
Is there any other common or better form for this question ?
Thank you for your helping.
That is fine as it is.
Context is always important; labelling is rarely important.
There is really nothing wrong with that question, as long as you are trying to find out why road signs exist and what their purpose is.
You could say "What is the purpose of road signs?" or "Why do we use road signs?" but they are no better or more common than your question.
Please see the parts I have marked in red in your post. Remember that we do not put a space before a question mark or a colon.