Personal markings/ bespoke painting of aircraft

For many years I've both read books and seen images of variously coloured WW1 aircraft, with many of them described or seen in all manner of customised paint jobs. In addition there seems no end of personal markings.How and why was this practice adopted, and how extensive was it. Is there a reference work that shows these? I've looked at the Herris/Pearson book, 'Aircraft of WW1', which appears to indicate this was a very wide practive.