A product allows you to pre-define charges by creating a template of details that can be uniformly applied across all your customers.
For example, if you sell handsets to your customer base and the pricing of the handsets is the same across the board, by creating them as a product you can quickly apply them as single charges as and when you need.
Likewise, if you were to set up a recurring SIP Trunk product, you can apply the product as a charge to respective customers without needing to fill in the details each time.