I primarily use them as surge protection. I have lighting/surge protection for the house, but add more for expensive equipment. Sound improvement? Haven’t noticed. YMMV. There are no a big RFI or EMI sources where I live and mains are clean. Nothing fancy, mostly in $200 range.
But effect may also depend on type of your equipment and how good are their power supplies. Some tube gear has pretty simply power supplies and may be more sensitive to the noise.