Ralph touched on the most important aspect of the builder's log.
An aircraft constructed in Canada is governed by our regulations which allow for the use of professional construction. The US regulations do not allow for the use of professional construction in the same manner.
For many builders the value of the builder's log is to document the extent to which professional assistance (if any) is used. This document trail then becomes valuable if one wishes to see the aircraft into the US market. An aircraft imported to the US from Canada will have to pass the 51% amateur-built test and the builder log may be the only substantial evidence a Canadian vendor can use to substantiate the amateur-built content of the aircraft.
Now for the more practical perspective...
I can't count the number of times I've gone back to my builder's log to check the part number or serial number of a component installed in the airplane. This, to me, is the biggest value of the logbook. Recording part numbers and serial numbers of components, measurements and settings, the logic behind a particular build decision (now why did I route that wire bundle over there?) and such details makes the builder's log a vital piece of information. It is invaluable in easing the burden of continued airworthiness of the aircraft.
Now for a pet peeve of mine - inadequate supplemental information recorded in the Journey Log.
The builder's log is not one of the documents which is formally required, but the Journey Log is formally required. For the sake of our future sanity it is always a good idea to record compliance with each and every manufacturer's mandatory inspection or modification, be it a Service Bulletin, Service Information Letter or similar. Having this data recorded in the Journey Log allows us to easily confirm compliance and to track recurring inspection requirements. Remember those little blocks down at the bottom of the Journey Log pages where we record things like ELT battery replacement and altimeter calibration as well as annual inspection and oil change schedules? That's also not a bad spot to write down and track mandatory recurring inspections.
While it's work to write this information in a logbook, if we're smart when we do it and make the information easy to find, the up-front labor investment will be handsomely rewarded by labor savings and frustration avoidance throughout the life of the aircraft.