Some went for equipment. Some for training. Some for supplies. Some for rare and valuable materials and articles and rare books.
My Wizard built a laboratory and a library. He also helped fund a school and a hospital. He built several houses and homes. He bought into a shipping and trade organization, so he could spend less time adventuring and more time doing research and creating his own spells and inventing things.
My thief bought into several businesses. He also created a series of hideout locations which he stocked and supplied. He also helped build a library, took up pawning items and running a rare items smuggling ring. He built a large home and occasionally he would work for the government as a spy-master.
My Ranger built a series of frontier outposts which he helped equip, man, and garrison. He also bought into the same shipping and trade organization as my Wizard, or which he took a share of the profits. He bought a ranch, and cattle, and horses and raised stock off of which he made money. He became a famous horse and dog breeder. He built roads along the frontiers. Eventually he bought a ship, took to sea-commerce, took up exploring more or less full time before he retired, and in his old age took to fighting pirates. Occasionally he would work as a bounty hunter and frontier's lawman and spy for the government, who built him a Keep and awarded him nobility and freedom from local taxes in exchange for supplying men for defense. He passed most of his wealth on to his children and grandchildren.
We still play games in which I encourage my players to spend their wealth for better purposes than immediate desires, to build up things and develop things, and to invest in businesses and to buy influence.
I always though the sort of Developmental Sims game within the game was one of the best and most useful things about early D&D.
But we have each individually and as a group used our equipment lists for all other kinds of things. Mainly we barter with treasure in trade for other equipment or services or both (like a craftsman to create an item). Prices of these are defined, but unknown to us. So we often overpay and spend more than we need to on things. Some of these are even utterly useless towards reaching our more heroic goals so to speak. But spending it on goofy stuff is all in the name of fun too. And we do generally learn what prices are through bartering in end, so it isn't a waste of time ultimately. 
