It's not surprising why Roku would go this route: Making its own audio products isn't exactly easy, and it also limits choice for consumers. With TV Ready, less tech-savvy consumers also won't have to worry as much about making sure all of their audio settings are correct.
So far, Roku has Sound United (the parent company of Polk, Denon, Marantz, Class A and Boston Acoustics) and TCL signed up for the program. Both Denon and TCL plan to start their integration in upcoming soundbars. And we'll likely see more supported devices throughout the year, since Roku says it's relatively easy (and free) for manufacturers to implement it in soundbars and receivers that support HDMI ARC.
Of course, nothing is stopping you from plugging in any other audio device into your TV. But I'm sure even hardcore audiophiles could appreciate controlling their systems right from Roku TV remotes.