Why is percent yield important to know for a chemical reaction?
Typically for financial and logistical reasons.
Lets say you are a company making Tylenol. If the process for making it has only a 1% yield, it means the process (while it does get you the product) is incredibly inefficient, meaning that both time and money are being lost. Because of this percent yield is very important for industries trying to make the most product with the least waste.
Another use for it would be in determining other products that were made. Often in organic reactions if a percent yield is lower than expected it means you need to go back and figure out if any other products are being made besides the one you want (or that you just messed up along the way).
While not super useful for figuring out chemical structures and the like, percent yield is helpful as an indicator that your method is efficient and working correctly.