Acoustic consultants rarely talk about soundproofing, because we don't much like the analogy with waterproofing. A thin layer of plastic is waterproof enough for many purposes, but getting enough sound isolation can be a real engineering challenge: the limit is usually set by the permissible floor loading.
Unfortunately, sound absorbers, such as foam blocks, are often referred to as soundproofing, so people think they will help keep out sound from the neighbours. Well, they will a bit, but usually not enough to notice the difference.
Studiospares, in general an excellent and useful company, are rather naughty in that respect. This product is listed in their catalogue under insulation:
STUDIOSPARES SOUNDPROOFING FOAM Inch-thick (25mm) fire-proof foam as recommended in Sound on Sound's recent guide to soundproofing your studio ...
Well, maybe I read the wrong Sound on Sound article. The one I saw seems pretty sensible, and says that you need good solid walls for sound insulation - not 25 mm of foam.
So why does foam help at all? Well, if you completely cover a wall with a layer of foam, any sound coming via the wall also has to go through the foam, and that does reduce it a little. More significantly, the foam absorbs some of the sound that is bouncing round the room, so if the room would otherwise be very "live", that can make a worthwhile difference.
(I'm talking about foam, but of course the same applies to other sound absorbing materials such as mineral wool and fibreglass.)
But adding foam (or other sound absorbing material) to a normal room will not usually make enough difference to the isolation to justify its cost. Of course you may well want extra sound absorption for other reasons, in which case any improvement to the isolation will be a bonus. But if you want a real improvement in sound insulation, you need something a lot more solid - and some good advice!