Product clearance or discounting is a method to sell stock that you want gone quickly, often at times it will be at a loss. This loss, if in a food and drink market is still better than throwing it out for $0 return.
Depending on what a product is can determine the means and methods of using clearance pricing and promotion. For food and drink you want it gone ASAP, and should be doing sales on these as they get near to expiry. On broken products or opened products you want these gone quickly too, and usually a swift reduction depending on the factors like: original price, damage and stock.
Clearance, wanting gone quickly.
Products that may have a new version/model/style coming in soon also need to be sold quickly. These can see a level approach to discounting. Starting low and getting lower and lower until a customer bites. Clothing and technology gets this done a lot, obviously because these are high dynamic and changing markets.
When a company goes into liquidation stock will also get sold in an attempt to recoup some money. The prices will be at or near what was paid in wholesale meaning the customers get some good deals. A nice example of this is when DickSmith closed, some very nice deals where to be had especially with display stock.