How to design a metal plate for PCB
These shields are used to deaden EMI/RFI both to the circuit, and from the circuit.

I have seen them most often used to contain RF noise so that the product passes regulatory emissions testing.

Also, these can help "modularize" the design.  One of the problems with RF circuitry is that the tuning changes with the surroundings.  Generally an RF-enabled product is tested while it's in the case/package that it will be used in.  If you change (or remove) the case, the RF path can go out of tune.  Isolating your RF circuitry inside one of these faraday cages can minimize the external influences on the product. 

The shields come in a large assortment of dimensions.  Here is a link to DigiKey's RF shielding products.  As far as customizing the case with your logo or part number: this is often done with a laser etcher, but a simple label is easier :)