I’m not an Epic dev, but hopefully I can answer that question:
A simple example would be an ability that adds magic immunity to a unit. The ability would apply a gameplay effect to the target that has a duration and grants a MagicImmunity tag. Other abilities can then be blocked if this tag is present on the target.
A more advanced example would be an ability that gives a target a gameplay effect that has a duration and grants an explosion ability. This ability can then be activated when the “OnFire” tag is applied. You now have an “oil” ability that can make things explode if they come in contact with fire (which could come from any number of sources, as long as they apply the OnFire tag).
Tags are very useful for building your game in a systems manner, where you only need to worry about conceptual ideas once and it allows interactions to occur dynamically and organically.