Tagging is important because it helps search engines to classify your hub. You should add your keywords and phrases there as well as related words and phrases that describe your article. If there are other articles with the same tags then your article will be mentioned on those tag pages as well (just click on any tag to see what I mean). That is a good thing because that means more pages on the internet are linking to your hub.
Adding too much tags can harm your article as that can be seen as tag spamming but I don't know the exact limits - it also depends on how long your article is. Longer articles are allowed to have more tags.