In a nutshell, some oil vapour gets recirculated through your oem pcv back to the inlet valve port. Some of these vapours settle on the intake valves and, over time, the crud you see in the above pic builds up.
What a "normal recirculating" catch can does is condense most of these vapours and store them within the can itself thereby reducing the amount of vapour reaching your inlet valves.
The race type catch can goes one step further by venting any vapour to atmosphere so no vapour reaches the inlet valves. Whilst this sounds ideal vs the "normal recirculating" catch can, you will most likely fail your MOT emission test plus you may also get an oily smell in the car with a light oil coating in your engine bay with it installed.
hth