Dear Syed,

powder is used to randomize the angular distribution of the crystallites in your sample in space. So you are able to access all reflections in your XRD pattern.

For nonhomogenous angular distribution you will not 'see' all peaks at their nominal strengths up to failure of peaks.

In other words, solid samples can have some preferred orientation (texture effect), due to which the observed peak intensities change (increase or decrease). If the texture is very strong you may find that some peaks are totally absent. But the powder does not have such problems, that is why it is preferred.