A large part of what's left over probably goes to service consumer debt, as the average American carries about $6k in credit card debt. Paying for yesterday's stuff makes it hard to save for tomorrow.
The hypothetical family I described above would easily end up with $6k of credit card debt after a few years on that budget, simply because $900/mo is not enough to cover all the remaining expenses and therefore they will need to tap a loan at some point. A single auto wreck, temporary job loss, physician visit, etc. can create long-term financial problems for this family because they don't have an extra $3k or whatever to cover a short-term expense.
(I take an agnostic point of view on the debt itself. The alternatives to not having some kind of financial cushion -- even burdensome consumer debt -- are often worse. Think: "can't get to work because the car is dead and I don't have an extra $750 so now I lose my job and then my housing" as the realistic alternative. The real problem is compensation has not paced inflation for 2 generations, and that the $100k family should be earning much more for the same jobs.)
