Your suspicion is correct that, at least in many properties, they are not strictly necessary. I believe it is more a traditional thing because I have seen lots of houses, mostly abroad, plus office premises where there was no skirting board in sight. raysparx points to one thing though, plaster walls - especially the old type - are susceptible to damage from cleaning efforts (brushes, vacuum cleaners, water, etc) and in such cases skirting boards are more or less necessary. In the old days the wall plaster stopped short of the floor and the gap was hidden by the board (chicken and egg as to which led to which ?) but modern plaster together with modern cleaning methods/habits make skirting boards mostly decorative.