What the thing is - Cylindrical container the size of your palm, these oil catch cans are designed in a manner so they 'trap' crankcase fumes and alleviate the crankcase pressure which could possibly cause engine seal leaks or damages. Modern combustion engines have a mechanism of optimizing the efficiency through all means; even sending in refined engine fumes to the intake manifold. The problem is, as the motor gets old, the fumes are no longer properly distilled before being re-used again, resulting in injector clogging and shorter motor life. With an oil catch tank installed, you prevent this sticky oil entering your intake.
What car noobs think of this - Glitzy engine bay ornamentation. Erm, that's it. And nothing else. As a result, they resort to getting the cheap kicks and fake knock offs out in the market which literally do nothing except drying off the wallet. Even after a lifetime of hundred oil changes, possibility of having any residue in the catch can is nil.
What auto tuners/ enthusiasts think of this - Worked out naturally aspired motors and engines running high psi forced induction have more crankcase pressure which needs proper release. Enthusiasts ensure to get the branded catch cans which actually function as opposed to (cheap) non-branded ones. HKS/ GReddy/ Cusco ones are around in eBay for around US$ 200. Pricey, but they work. A cleansing of the tank is required occasionally, depending on the car's make and model; some say after every couple of oil services.
Or if you want to be a rebel, just get a breather worth nothing and put them on.