The 18th key is usually a left hand Eb/Ab lever. This is an alternative to the right hand Eb/Ab lever and makes fingering combinations involving both fourth fingers much easier. It's become standard on higher quality instruments.
The 19th key is normally found only on so-called 'full-boehm' clarinets that have some other extra keywork as well. This gives you an extra note, the low Eb, which is sometimes useful for transposing A-clarinet parts. For most players the advantages don't outweigh the extra cost, weight and complication.
In the case of the 'Tosca' clarinet you've linked to, the extra key has a different function: it opens a hole to improve the pitch of the lowest F which is often flat.