Maltodextrin adds body and improves head retention (but doesnt add alcohol).
or flavour.
found the amounts years ago, makes life easier knowing whats what.....
be1= 600g dextrose, 400g maltodextrin
be2 = 500g dextrose, 250g maltodextrin, 250g light dry malt
Mixing this up yourself with ingredients from the lhbs is much cheaper than buying the branded stuff....and you can tweak it to how you like it. If the malt is increased enough, it adds enough body that you don't need the maltodextrin at all....but you can only increase the malt so far before you need to add more bitterness with hops.
Kits, for the most part, are designed to be used wih around 250g malt in your adjunct mix. Depending on tastes, you can get away with a little more before you need to add hops. (for example, coopers tins are designed, for the most part, to be used with a coopers BE2 and nothing else, in its bog standard form. Most other tins follow this example. Morgans tins, on the other hand, are designed with the intention of using one of the Morgans Masterblends with it...and these have much more malt. so the Morgans tins are hopped to a greater degree than most other tins.)
When I used tins, I always considered that the manufacturer intended it to be used with 250g malt....so when I upped that to a kilo, I would hop the right amount to offset the additional 750g malt I was using.....bringing the level of bitterness back to what the manufacturer intended in the first place.