All of Bethesda's GOTY versions come with all the expansions that were released for that game. For example, Fallout 3 GOTY comes with Point Lookout, Op: Anchorage, The Pitt, Broken Steel, and Mothership Zeta. When the expansions were released they cost $10 a piece, in addition to buying the main game for $40-$50 if you did not have it already which would total to around $100.
With the GOTY version you get it all for $50-$60; as a bonus you get the smug sense of satisfaction that you patiently waited for it to be released as a set, instead of blowing all that cash on a quick fix every couple of months.