The Convert to gray filter transforms the coloured image into the gradations (hues)of gray image. It transforms the image so that in the result of the transformation levels of one, in that case – gray colour, were different only in brightness (intensity).
This operation of transformations of coloured images and photos is often needed in order to apply filters which work only with monochrome images: halftone or binary. For example, morphological operations (Dilate, Erode, Opening, Closing, etc.), threshold transformations, Laplace, Sobel , etc.
Also, when the use of the filter is effective for economizing resources by keeping the images in the cases when the colour is not very important (full-coloured images, as it is known, occupy great part of memory).