What are the uses of fan shaped brushes and finger shaped brushes?
The fan-shaped brush is perfect for blending oil paints. I use it to either blur fresh paint (be it an area or a detail) into the background, or to create a smooth gradient between paints.
When painting with very fatty paints, or simply a lot of paint, this brush will lose its efficiency, as it is too thin and fine to manipulate, accumulate, or contain much paint.
I have no experience using this with acrylic paints, but can imagine it will work similarly, when using little and diluted acrylics.
With this brush it is especially important to clean it regularly, as it loses this functionality whenever there is the slightest irregularity (leaving obvious marks in the paint surface).

With the comb, rake, or fingered brush I have a slightly similar experience: it can be used to blend areas or details while maintaining a certain amount texture (resulting in a 'broken' gradient, if you will).
It's also an obvious contender for painting details like vegetation (grass, reeds, branches, &amp;c.) and hairs, as it naturally leaves lines.
It can also be used for graining or painting imitation wood (although those brushes are usually larger).