Personally, I dont think they do anything at all other than look weird. (My wife hates them, but she wont let me have a cat so I'm punishing her).
The sand bags may change the sound but not necessarily for the better. A lot of speakers are designed to have 'live' sounding cabinets and it's one reason why mdf is used less than plywood in speaker cabinet making.
I have mass-loaded my cabinets though with 12.5 pounds of lead in each. I think the change was very modest, not really noticeable to me (tin ear maybe).
The sand bags may change the sound but not necessarily for the better. A lot of speakers are designed to have 'live' sounding cabinets and it's one reason why mdf is used less than plywood in speaker cabinet making.
I have mass-loaded my cabinets though with 12.5 pounds of lead in each. I think the change was very modest, not really noticeable to me (tin ear maybe).