Old actor tags or component tags are just strings. Unsafe to use (like any other string field where changing value often leads to bugs) and providing no features. These tag fields are just dinosaurs from the bygone era. Generally, avoid that.
Gameplay tags are great, stored in the “central location”, in .ini files. And once a tag is added, you can access it anywhere in the engine. UI doesn’t allow to simply change a name (preventing bugs), although there’s no easy way to rename used gameplay tags, sadly.
Gameplay tags support hierarchy, a tree of tags. Extremely useful thing. Just read docs and embrace it 
https://www.unrealengine.com/en-US/t…content-in-ue4
https://docs.unrealengine.com/en-US/…ags/index.html
More precisely, it was probably added for the Gameplay Ability system - initially developed for Paragon.
https://docs.unrealengine.com/en-US/…tem/index.html