Tags are used to describe the topics that a paste contains. Taging is crutial because it can help readers who want to read something of some kind. If they do not know your work already, this is probably how they will find it.
Tagging is, by all means, not a fully developed science. You can fill the tags with what you think is correct. If you think you tag more/less, you might want to get a second opinion about it, because most of the time these are very specific issues.
In the search page you can place the tags you want to search. Pastegasm is indeed like a booru, but for text. You can write one-word tags as-is, and [multiple words tags like this] with brackets.