I'm not the guy you're responding to but for me, GnuCash has helped me identify a number of subscriptions I don't use anymore, that I have then cancelled. It also helped me identify a direct debit that I had forgotten to cancel when I moved house. So there have been tangible savings. Of course you could argue that I could have discovered these things just by reviewing my bank statements regularly, but that's most of the work of importing them into GnuCash anyway, and it's easier to spot these things when you are actively categorising your transactions.

It's also just a helpful way to track the growth of your net worth over time when you have multiple accounts, investments, etc. You may or may not take action based on what you find but it's good to know.

I actually do exactly the same thing, and the value for me is that my actual income statement, balance sheet and cash flow are just much more accurate than other methods and therefore improve decision making (e.g. in a previous job I paid for work expenses on my personal card and was reimbursed from work, other solutions would make it look like I had was spending and earning an extra ~$50k-100k/year Vs with gun cash I just booked it all to a balance sheet account, which accurately reflected reality)

I run reports monthly to see where I'm putting my money. I used to follow a strict budget but that was way too rigid for me and would sometimes put me in a rock a hard place. So now I just spend as I'm comfortable and adjust in the future months.

If you own a business or do work on the side where you are 1099'd then it makes it way easier to send your books to your accountant so they aren't asking for information.

The answers to that are always going to be pretty personal. To me the small amount of time it takes is well worth it to have full insight into where money is coming and going.