Tags are also known as "keywords". This helps to drive in the traffic to your page. So, we should be wise in choosing the right keywords or tag.
For example, if your article is about painting, that should be considered as the main tag, and other possible tags should be paints, colours, brush, painting cloth, types of paintings, water paint, oil paint etc. Hubpages use this information to get your article on the list when someone search for it.
It also helps you to get enlisted in other listings inside hubpages.