Layer > Matte In Transparency works similarly to Layer > Reset Transparency, in that it also makes an entire full-color layer opaque.  But, it tries to preserve the current appearance of the layer, by essentially painting the transparent areas with the frame's background color.

This is particularly useful when you've drawn with the drawing tools on a color layer (or you've converted a spot-color layer to a full-color layer), and you want to start working with transparency.