need for an extra variable.

hello everybody,
consider the following code snippet (from K & R, pg78)

while ((s[0] = c = getch()) == ' ' || c == '\t')
;

my question is, why would we need the variable 'c'.
We could easily write it as,

while ((s[0] = getch()) == ' ' || s[0] == '\t')
;

.