Full Idea
Sense-data have an epistemological purpose (to serve as foundations on which the edifice of knowledge is to be constructed), and a metaphysical purpose (to provide an accurate account of the phenomenology of perceptual experience).
Gist of Idea
Sense-data have an epistemological purpose (foundations) and a metaphysical purpose (explanation)
Source
Barry Maund (Perception [2003], Ch. 6)
Book Reference
Maund,Barry: 'Perception' [Acumen 2003], p.116
A Reaction
This is very important, because there is a real danger (e.g. in Russell) that the epistemological convenience of sense-data for giving reliability in knowledge means that we are too quick in making the assumption that they actually exist.