As any serious macro or close-up photographer will tell you, a dedicated macro flash is an indispensable accessory.
One of the main challenges when shooting up close is that as the working distance decreases, so too does the light that reaches the camera's sensor. When using a macro lens at 1x magnification, there's around a two-stop exposure factor to consider, for example. While that's not much of a problem at the maximum aperture, maintaining a high enough shutter speed is more difficult when maximising the depth of field with smaller apertures. And at greater magnifications, there are even larger exposure factors to overcome, due to the closer proximity of the subjects.
Introducing additional lighting for macro photography is extremely useful, as it allows photographers to use a fast shutter speed while also stopping down the aperture to increase the depth of field.