Halliday & Matthiessen (1999: 169-70):
This construction, Process + Range/Attribute, is much more common [than] the agnate form with Process only (that is, ‘be + important’ is the preferred model rather than ‘matter’). The Process just embodies the category meaning of ascriptive being — ‘be a member of’ — and the range carries the specific information about the experiential class. It is interesting to note that the ranged construction sorts out the ambiguity of the present tense between habitual (doing) and occupational (being): she dances/does a dance every night, she dances/is a dancer (by profession).