Halliday & Matthiessen (2014: 395):
Those of variety include a few very general terms such as kind, sort, type; these are the expressions that have led to the use of kind of, sort of as little more than markers of hesitation in casual speech. The words example, instance, specimen sometimes function in a similar way, especially in exemplifying identifying clauses, e.g. a toad is an example of an amphibian, whereas with a word like kind the meaning of classification is embodied in the clause structure and is construed by the clause structure even where the classifying word is omitted (a toad is an amphibian).