A frame that has cracked or given way
If the sofa creaks under you, leans, or one corner has dropped, the problem is the structure rather than the seat base. A board laid across the seat will not straighten a frame.
Springs that have snapped rather than stretched
Webbing and springs that have slowly gone slack are exactly what this is for. Metal that has actually broken needs a repair, and the board would sit on top of the fault rather than take its place.
Cushions that are worn right through
The board gives your cushions a firm base to rest on. It does not put thickness back into foam that has collapsed. If the cushions themselves are thin and flat, you are looking at new cushions as well as a board.
A sofa bed
It helps with the sag, but it cannot fully cancel out the folding mechanism underneath. Go in expecting an improvement rather than a fix.
You like sinking into your sofa
This makes the seat firmer. That is the whole point of it, and it is the wrong buy for anyone who wants soft. We would rather say so here than have you send it back.
Not sure which one describes your sofa? Take the cushions off, send us a photo of the bare seat base, and we will tell you straight.