It's a way to turn money into combat power at the higher levels, once you run out of things to buy. If this were the 3.5 economy, it would be silly to invest large amounts of money into a minion who was that far below you, but 5th ed is different.
First, as @Aviose noted, you run out of things to do with your money. At higher levels, you need to look for things to spend money on. You might want a massive vault of gold to swim in. You might want to build a castle, or buy in to a major metropolis as a merchant prince. Alternately, perhaps you want to have a smallish army of golems. Regardless, the monetary cost isn't actually reducing your personal power all that much.
Second, "small army of CR5 golems" actually is a meaningful force on the battlefield, even at particularly high levels. They'll have enough HP that they won't just go down all that easily, and bounded accuracy means that they'll each still have a chance (not good, but not terrible) to hit the sorts of enemies you're likely to be facing. Figure out how to use them strategically, and they could be quite helpful.
By comparison, conjuring a CR 5 elemental takes a 5th level spell slot and your concentration, and only lasts for an hour.