Unlike machines, no human being is fully productive and creative 24/7.
So, if you are a customer of a law firm and for some reason you are silly enough to allow your law firm to bill you by time, at least ensure that the lawyers working on your matter only work on your matter when they are at their most productive and creative best.
This will mean those lawyers are free to work on the firms’ other customers’ matters when they are less productive, a bit tired, slow to get going, day dreaming, a little disorganised, unprepared, or just having a bad day.
* "Oldlaw": firms which business model leverages people x time x hourly rate.