What is the purpose of adding keyword tags inside a keyword?

There is a practice, existing before and outside of Lightroom Classic, called a “controlled vocabulary,” where a structure of topics is standardized so that when different people organize or look up information in the same resource, they all understand and consistently use a standard hierarchical structure used to organize the information. The controlled vocabulary can be anything from library books, to biological species, to product categories in an online store.

There are people who download and buy controlled vocabulary keyword lists and import them into Lightroom Classic, so that they don’t have to make up an entire keyword hierarchy from nothing. They just import the keywords, and then they can simply follow a standard hierarchy of keywords they know other people have already used and perfected.

More info:

Keyword Lists / Controlled Vocabularies

United States

     New York

          Buffalo

          Rochester

          Syracuse

Birds

      Raptors

            Kite

            Owls

            Vultures

    Parrots

          Cockatoo

          Rose-ringed Parakeets

and a gazillion other examples

The real world is hierarchical. Keywords are (can be) hierarchical.