Fair point, sabreuse.

For me, though, that path leads to insanity. If I spend too much time reading the web, posting to productivity forums, etc, and at the end of the week I find I've achieved nothing for the company that employs me, I might have to make excuses, or lie, or work overtime to get a reasonable amount finished. That makes me stressed, so I choose not to.

Ultimately, your boss expects you to be delivering at least your wages in value - (s)he expects to be able to sell what you make for more than $X/hr. If you aren't delivering that value, you're not worth employing. It could be you can deliver more in less time (say, work for an hour and deliver $2X, goof off for an hour at $0), but you ultimately have to be delivering more than you get paid or it's irrational to pay you anything.

If you're self-employed, it's a guide to figuring out how you should be spending your time, and what's efficient to delegate or outsource. If doing the laundry takes 30 minutes, and your attention is worth $20/hr, then you should pay a neighbour's kid <$10 to do it and do some more work on your business.

A similar number is what I call 'disposable hourly rate'. Take your net income after tax. Take off all the boring stuff like rent, bills, groceries, etc. What's left is your disposable income for the month. Divide by your hourly rate. Eg, $2000/month becomes $800 after bills etc. If you work 160 hours per month, your disposable hourly rate is $5/hour. So you can shop and convert prices to time: this meal out costs me an afternoon's work; this new phone costs me a week of toil, etc.

They're just rough numbers to help think about priorities and choices.