Maybe the motivation for some ordinary folks (that don't really need structured accounting, then it might be great) is different than mine. Or if I would be on a really tight budget combined with having to juggle tons of various mandatory expenses (household, kids, whatnot).
For some things those will be fixed monthly expenses (rent, insurance, mortgage. Others will be variable, like grocery shopping, maybe clothing. And others will be bigger purchases like a computer.
It's just another framework or set of tools for budgeting your money out into the future. YNAB (You Need a Budget) has some decent articles on their blog about the concept. Dave Ramsey might be helpful too if you really want to understand the motivation behind this kind of system.
Yes, and envelopes make it easier for many people to figure out if they have the cash for it. They let you "spend" the money the instant you get it, in a virtual way, even if you don't really need to spend it until later. This way you know that when it comes time to pay rent/mortgage, property tax, etc., you're good.
I think you are right though: I think the envelope idea is primarily targeted at people on a tight budget, where you really are trading off pros and cons of spending relatively small sums of money, and where the usual question is what gets sacrificed if this or that is purchased.
You suggest that in such a situation you'd look at the root cause of such behavior (e.g. compulsive spending). Often, the "root cause" is simply unawareness of spending patterns, and envelope spending is a useful technique for a lot of people. It isn't a crutch to cover up some sort of underlying personality flaw that has a deeper "root cause" -- It is just a way of tracking earmarked spending.
Root cause analysis is of course best if successful and can be completed quickly enough.