As their name suggests, pre filters are responsible for filtering out the particulate in air before it gets to the HEPA or ULPA filters. Generally, these remove larger particles. Using pre-filters is also a great way to extend the life of your HEPA filters, as it makes the HEPA filter work less.
If your filtration system has pre-filters, it's good practice to change them about 6 times a year, or every other month. This will keep everything in your filtration system running smoothly, and will ensure that none of the cleanroom system parts past the pre-filters experience excessive contamination.