Even if you bill by the day, that is still a certain number of hours to you.

What he is talking about is how difficult it is to work enough "hours" when you are contracting. If you work for a company they pay you to be at your desk 9-5, sure. But in that environment 10 minutes talking to a co-worker as you get a coffee, 15 minutes trying to get the damn printer to work, 5 minutes dashing up to the next floor because the toilet on this one is busted... doesn't matter that much.

When you work as a contractor targets are much more immediate - and if you don't get a days work done on a project that starts to pile up. It's crazy how little things can stack up; I didn't get "enough" done today, really, because the coffee machine in my kitchen was playing silly buggers first thing, and I got distracted fixing it (I know...). That took half an hour off my day and meant I had to try and make it up this evening... but with one thing and another...

I imagine it takes some time to learn to properly estimate how much to charge for a project. Until then, billing by the hour is a way to build heuristics.

completely agree, I recently switched to billing by the day for new work and so far it's been a lot easier and my clients seem happier. How often do you really get a task that can be completed in less than half a day (and if it could, couldn't it be lumped together with other smaller tasks that together would take at least half a day)? Billing by the day, or at the minimum half day means I have at least that amount of time to dedicate to a client without jumping between tasks and getting distracted. I still break down projects and estimate tasks in hours, but when presenting estimates for projects as a whole, I just use days. It's more meaningful for clients and it's better for me.