Why is the equation of a progressive harmonic wave given as y=asin(wt-kx)? At each point on the wave, wouldn't the value of (wt-kx) always be 0?
Set x=0 (this means you are looking at the point in space where x=0).
Then y(x=0) = a sin(w*t) . So at this point, it is clear that the value of the quantity you are observing will vary periodically.You can perform a similar reasoning for every point in space.
Actually, you can also perform a similar reasoning by setting the instant t and observing the "shape" of the wave in space.