In WOIAF, it mentions the need for a “Heir and a spare”? What typically happens to the spare, and the spare’s children (assuming that the heir has son(s) of his own)? I.e. what would have happened to Tommen and his kids if Joffrey lived a long life and fathered sons with Sansa? Or to Viserys if the Targaryens stay in power and Rhaegar/Aegon live? Unless the second son marries a Dornish princess, they won’t really get any land from their marriage, no?

Well, a spare usually gets a lesser title and some lands. For example, Viserys was Prince of Summerhall, and Tommen would have probably gotten that too. And recent practice shows the spare often gets government office too – Daemon Targaryen as Lord Commander of the City Watch, Stannis and Renly on the Small Council, etc. 

And they’re useful in times of war as guaranteed loyal commanders – look at Maekar and Baelor during the 1st Blackfyre Rebellion, or Maekar, Aerion, and Aegon during the 3rd, or Stannis during Robert’s and the Greyjoy Rebellions.

But mostly they’re there for dynastic alliances (and you can get land from marriage if you marry an heiress with no male relations) and in case the immediate royal family kicks it.