enthalpy - What is the significance of the "Standard Temperature" in Standard Enthalpies of Formation tables?

In my data book, there's a list of common compounds and their molar enthalpies of formation -- at 298.15 K. What's the meaning of this given temperature value (is it the final temperature of the compound in question?) and how does it affect the molar enthalpy of formation of a substance?

What, for example, does it mean when the molar enthalpy of formation of H₂O(g) at 298.15 K is given as -241.8 kJ/mol, even though water should be in a liquid state at 298.15 K? And what would the molar enthalpy of formation of H₂O(g) be at, for instance, 500 K?