Pre-filters target bigger pollutant particles like dust and pet fur. They exist so that the HEPA filter can have a longer lifespan!
Pre-filters may also be somewhat useful during the pollen and allergy season. Tests Smart Air ran on the Smart Air Blast and Smart Air Blast Mini pre-filter found that the pre-filter could capture about 5-10% of 5µm particles. Pollen particles range in size form 10-200µm, so a pre-filter may be able to capture some of these and help increase the HEPA filter lifespan.