There are other advantages as well. If you log each purchase and categorize it (which is easy to do in Quickbooks or other programs, which "remember" what category you used for a merchant the last time) you can run reports at the end of the year and see how much you are spending on cars, food, restaurant meals, liquor, hobbies, electric bill, and so on. It often is a startling amount.
We all like to lie to ourselves about money and other things. As I have noted before, men lie about two things primarily - their gas mileage and penis size. I see men with gas-guzzling motorhomes tell me they are getting 15-20 miles per gallon. The size of the lie about that tells me their penis must be particularly small.
But we all do it - we round down on expenses and round up on income. We don't want to really think about how much our hobby car is costing us, because we'd just weep at the waste of it all. I recently bought a $299 golf cart "buggy" and so far we have nearly $1500 into it with a new controller. I talk to other people with fancier carts and they say they spent $5000, when clearly they have closer to $8000 or more into it. For what they could have spent buying a secondhand car, they bought a golf cart. And no one wants to think about that!
I use Quickbooks as an example, only because I have a "paid-for" copy of the software from many, many years back (No, you don't need to "subscribe" to software or update it every year). There are other programs out there you could use. The main point is to reconcile your accounts and to categorize spending. When you do this, you understand "where all the money went" and you realize how small expenses can add up.
For example, restaurant spending can be considerable. I log this and am chagrined to see we spend as much on restaurant meals as we do on groceries. And we do not eat out very much, either. Some folks eat out five times a week or more - folks I know. As a result of this awakening, I try to eat out far less often than I used to, and when I do, it had better be more than a refueling stop. I also try to look at the cost of menu items and split an entree when possible (also better for your health as well) as so many restaurants today have huge portions, and it makes no sense to spend $15 on an entree and leave half of it behind, or worse yet, take it home in a clamshell.
Logging spending made me realize how much I was squandering on cars as well. But that category illustrates where logging your spending might not give you a complete picture of overall costs. Depreciation is not shown on your Quickbooks accounts, so you don't realize that you are losing thousands of dollars a year on a car that is just sitting in your garage and depreciating - costing you money even if it is doing nothing.
You can't control that which you don't measure, as one reader put it. And measuring spending, on an item-by-item basis, is key to controlling spending and accumulating wealth.