Combinatorial is popular for spectrum auctions as well. Typically it's used in auctions where there are different bands of frequencies being sold at the same time (700Mhz and 2.1 GHz in the same auction for example). In that situation, bidders bid until supply meets demand to determine block prices in each band. Then the bidders can bid any combination of blocks less than or equal to what they bid at any point in the auction and give that package an overall price. The solver mechanism crunches all those packages to determine a solution that maximizes revenue for the government.