The Game of Life is usually played on a very large field. WOPR's 16x16 grid is tiny by Life standards, but this gives it some interesting properties which make it particularly useful for modulation purposes.
The grid wraps. The top and bottom edges are joined, as are the left and right sides. (The grid is effectively the surface of a torus, laid flat.) That's useful for a few reasons: things like gliders can travel diagonally across the grid forever, rather than disappearing off into the distance. Two or three parallel gliders can set up waves of modulation over the matrix, guaranteed to run forever.
Wrapping also leads to interactions between cells at the extreme edges, something which just doesn't happen in regular Life. For example, try a right-facing Queen Bee near the left edge of WOPR's grid. The gliding shape and the debris at the left interact thanks to the wrapping, which eventually leads to a large, period-3 oscillator which is used in some of the factory presets.