Why is it important?
Well, my dear, placing objects (type, image, color, whatever) near each other can have dramatic effects. The position of said objects can give the composition a stronger meaning than each individual object on its own. (The whole is greater than the sum of its parts.)
When unplanned juxtapositions happen, they can create unexpected, surprise results. Sometimes these results are pleasing, and sometimes they are not. Harnessing the power of juxtaposition by purposefully placing objects together can become a very strong force by giving the viewer another layer of conceptual depth within the composition. Objects can reflect upon each other to form a sense of unity, or conflict with each other to create a sense of tension and unease. When objects are further apart, this kind of relationship doesn’t happen, and they are just objects on their own without any context or partnership.