Crystallinity is a broad term that refers to the overall makeup of just about any type of solid compound. Crystallinity covers the spectrum from the hardness of a compound, to the creases and crevices found within it, to its density, and even its color and clarity. But additionally, crystallinity is only present in solid compounds whose chemical makeup makes sense - both in terms of structure and organization. Imagine a drawer full of socks: socks that are arranged in neat pairs most definitely have crystalline properties, since it's easy to distinguish one pair from another. The arrangement of the chemicals that make up crystalline compounds are referred to as polymers; and it's the examination of these polymers that enables scientists to correctly identify and categorize various types of solid compounds.
Of course, there are plenty of solid compounds out there that may not be entirely crystalline, because they have inconsistently-arranged polymers inside of them. These compounds can be a little trickier to identify, but various types of scientific techniques, including X-ray, X-ray diffraction, and plain old-fashioned examination under a microscope will all help to provide the correct label.
Most people consider an X-ray machine to be used for checking for broken bones, or for security purposes in court houses and airports; but X-ray machines are also used to find out more information about various types of chemicals. Crystalline compounds, for example, play very nicely with X-ray technology. X-ray diffraction in particular is a preferred method of using X-ray equipment to correctly identify and evaluate materials. With the diffraction method, charged electrons are targeted at the compound that's being examined; the resulting charges given off by the compound will tell the scientist all about the material.
To summarize: crystallinity is an important component in learning more about organic and inorganic compounds. Percent crystallinity in particular helps with determining the percentage of a compound that has crystallized - in other words, the percentage of the compound that has arranged itself into neatly structured polymers. Scientists rely on X-ray diffraction to help with gauging the percentage of crystallinity, which in turn assists with identifying and categorizing all sorts of compounds.