Tags are to.. order things, to easily find what you need and create groups of similar objects. But internuts often get bedazzled and not only "don't" choose the best/most suggestive use of tags, but go for stupid ones.
Like.. let's say you were writing 20 stories about bears. The most useful tag that should unite those in a blog of about 2000 posts is "bear" or ... "stories on bear", "abearations" -or something similar.
But no. Looking at blogs i see tags like - one story has "Martin" [as a name of the main bear], one has "brown bear", one "Jimmy" and so on. That means 20 tags. ... Not smart.
Or i see something like 13 stories tagged "bear", 4 tagged "baer", 2 tagged "bears", "critters of the bear type" or something like it. It's a computer, numbnut :P It won't see that "bear" and "bears" are the same [...yet]. You end up with a list of 5 tags.
Duuh.