The Insert Block is used to improve dust collection by reducing the gap at the bottom of the dust collection area of the saw.  Your saw can be operated without the insert block in place while you are waiting for a replacement.  The tighter the opening where the blade exits the dust collection chamber, the better the dust collection.  Since the insert block is used in very close proximity to the blade, it is designed of plastic so that it does not damage the blade should the two items come into contact.  The insert block, like the blade, are considered wear items.  The insert block, like a Zero Clearance throat plate on a tablesaw, will get worn over time and is designed to be replaced occasionally.  This part is inexpensive and is available on our website.
Users can also make their own replacement insert block from a piece of plywood or MDF.  Should you create your own insert block, cut the block to size and then with the saw unplugged slide the block into the groove while rotating the wheels by hand so that the blade can cut the slot while the block is pushed into its groove.  Use pliers to hold the block while cutting the groove to keep your hands and fingers away from the blade.