Double-entry accounting models the source and destination of funds. If you keep all of your money in one bank account and neither owe anyone or have anyone that owes you, then double-entry accounting is not necessary because you can record just the one side of every transaction – your bank account is the other side.

However, if you have more than one account or debts, or budgets, then it becomes useful to track both sides of a transaction. If you track both sides of a transaction then you can easily answer questions like: 1. how much money do I have in my investment account? 2. how much debt do I have? 3. how much did I spend on recreational social activities?

You can track debts and investments separately, but then you are still making two entries just in two places.

I think many people have few enough debts and investment account that they track these separately, and the third question, budgeting, can be simplified if each vendor is only ever considered for a single budget, for example, all your flights are part of your vacation budget, and you don't care to break out a flight to a career related conference.