Essentially, scoring full marks on this question requires a response that explains both washing and drying. Based on this, here is a sample response that I believe may help:
This is a gravimetric analysis and requires accuracy. If you weigh the filter paper while it is still wet the result will be inaccurate because the mass of the precipitate will be inflated. Therefore it is essential to make sure that all the filter paper and precipitate are 100% dry. You must also wash the solution thoroughly to ensure that all the precipitate goes through to be collected, otherwise the recorded mass will not have all the precipitate recorded.
I hope this helps!