The VALUE is in the features a preamp provides. There are MANY different types and features. To me, the 2 main features are Input Switching and Gain Control.
Now of course if you have only 1 source and it has volume control, then by all means try it without a preamp. It may sound perfect. But it also might sound even better with a good preamp, or maybe not. It depends.
But most people need features. Like me, they need input switching, balance control, tone controls, headphone output, etc. It’s not a conspiracy. A preamp is a tool that does a job, and in some cases, you don’t need one.