What is DC offset on the AC power line?

I realize this question may be more appropriate in the miscellaneous section so please accept my apologies in advance. This may be somewhat amp related though, as in "don't use it with an amp".  

Can someone comment as to the pros/cons of a  DC offset suppression device, such as Emotiva CMX-2.

https://emotiva.com/products/accessories/cmx-2

I haven't seen this feature in other power strips and/or surge suppressors.

Thanks.