Tags are meant to allow people to follow activity on surnames or activities they are interested in. Everyone who is following the Johnson surname will be notified when some activity occurs on that name, but not everyone on WikiTree.
For example, several surnames I follow are Strutton, Luttrell, Stone -- so I get notified whenever anything happens on one of those names. This allows me to contact other researchers with common interests. Of course, many of those changes will not affect my family lines so you just have to tune those out and concentrate on the ones that are related.
WikiTree is a collaborative site. One of the things that helps us work together as a community is the ability to locate and work with those members who are working on the same families. Tags for followed surnames is one tool we can use.