Question: For some reason I feel its overkilling on the use of article(the) here, 

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> To refine the results, please use the categories on the left
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Can someone help?

Answer: 
@shin is correct, the sentence is fine. **To refine results, please use categories on the left.** is equivalent, but the difference is stylistic.
Technically, the sentence is correct but the is overused. I would rearrange the sentence to say "Refine results using the categories on the left". It removes a couple of words and also makes it a more active sentence. Also when writing instructions, avoid using please. These are instructions for performing a task and not a request so the word please is not necessary.