Koen, Welcome to the precancel world. Precancels can best be summed up in one sentence. Precancels were primarily used for bulk mailings or as some call it "junk mail". Catalogs, advertisments, mass mailings. Sometimes you will even see SRC or MW with a date on a precancel. SRC was Sears and Roebuck Company and MW was Montgomery Wards, two large mass merchendisers with big catalogs. Seed Companys were another big user of precanceled stamps. Nowadays its called pre-sorted and not very many stamps are used :( its all preprinted envelopes or metered. Once you get into precancels there is no turning back, its a fascinating collection to work on.
At least in the US, a lot of current precancels are not cancelled at all. But a lot of mailers do use stamps, because a letter with a stamp on it is more likely to be read than one with a permit box or a meter.
There are no current precancels, they are not allowed on mail anymore. The PM I believe ordered all precancel devices destroyed. But I see your point that if some advertisers want there mail read they will use first class stamps. Unless your talking those pre-sort stamps?
Knowing if it is a precancel or not is sometimes not easy. The simplistic answer for US precancels is "if it is listed in the PSS Town and Type catalog then it is a precancel". Many of the older stamps were questionable. Research and sometimes contentious discussion has been done over the years to arrive at the 'biblical' status of the T&T catalog. Newer editions of the T&T may change to reflect current knowledge re what should be added or deleted. The next edition of the T&T will be adding about a hundred new items and dropping a handful. Things are not that complicated in reality - usually if it looks like a precancel it is a precancel. If it is not listed in the T&T - you can probaly find it in the new Cummings book on fakes and lookalikes.