To add to what Scott has said, if you paint a model in three color camouflage those three colors often stand out on the surface. The demarcation lines between each color are very sharp and pronounced, the colors often bright, loud and vivid.

This has a negative effect on the scale look of the model. A filter applied over the entire surface can tone down these colors, soften their effect and give them a much more in scale appearance on the surface. While you want a wash to pool up in the nooks and crannies around raised details, you want the opposite effect with a filter. You don't want it to collect around raised details at all, you want a super thin, translucent color to cover the large surfaces.

Often I see the word depth thrown around in every effect but it really only applies to a couple effects, the filter being one of them and really, the biggest of them. This translucent layer affects the way light hits the model and reflects off it, shifting the underlying color as well as softening and subduing them at the same time. It's these translucent layers that actually give a paint job depth, as the light travels through them before reflecting back, reaching our eye and tricking our brain into seeing the paint job as having scale thickness in miniature, so your model looks like a miniature vehicle and not a small plastic toy.

They aren't always the easiest thing to master and take great practice. You basically take most of the color off your brush and wet the surface with that brush, sometimes a few layers are needed. Taking time to slowly build up the filter usually produces the best results.