There are many possible reasons for needing a patent search. Some examples are:
1. an inventor would like to make sure his invention is unique before he spends time and money to obtain a patent (this would be a Novelty search).
2. a company would like to produce a product, and although they do not wish to patent the product themselves, they want to make sure that they do not get sued by someone else that might have already patented the idea (this would be an Infringement search).
3. Company A is making a product, and is being sued by Company B, which believes it holds a patent on the product, and Company A would like to prove that Company B's patent is invalid (this would be a Validity search).
4. a researcher would like a comprehensive listing of the most recent patents in a certain field, to help guide his research (this would be a State-of-the-Art search).