As someone above mentioned, it is important for pilots to know the dewpoint in order to be aware of the possibility of icing during flight.....although the temperature at the surface may be well above freezing, it wil not necessarily be so at altitude, and if the temperature and dewpoint are close together, a couple of thousand feet in altitude can place you in icing conditions..........with potentially disastrous results.
Weather forecasters use it more universally than RH for purposes of evaluating changing conditions in airmasses for modelling purposes........dewpoint is a much more specific measurement for atmospheric sounding purposes.
It is also useful in agricultural applications in determining the potential moisture content in grain crops if it is plotted for a specific area over a period of time, particularly at this time of the year (harvest time).
doc