Direct light has an uninterrupted path between the source and the target. If the source of the light is a point source, as is the case for LEDs, this will cause a light spot or brighter patch in the middle of the object which is normally not desired. While this can be compensated later in software, it is easier to produce even illumination from the start.
In this diagram (right side), light has passed through a translucent diffuser which softens and disperses the light, creating a more even illumination. The only down side when using a diffuse light is that the intensity is often reduced.
Most suppliers of illumination products for imaging purposes will supply different versions of their lights either with or without diffusers.