because it fits with other uses of an 'area' model for multiplication
and seems effective in terms of conveying some rationale for the process
however, there are a few weeny complications with it
technically, a fraction of a length multiplied by a fraction of a different length is a fraction of the area of a rectangle, so for the first picture:
to a certain extent the complication is simplified by having fractions of unit lengths (in a unit square, making L = 1 and W = 1 and so LW = 1)
this is an approach taken by the helpful:
nzmaths.co.nz
learning objects
fractions of fractions tool
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