Metadata is the agents, elements, and attributes that allow your media to be descriptive and searchable.
If you only provide a few categories “agents” of information your users search will be limited or too broad. The user may not be able to find what they need if the information they are looking for is not an agent your metadata collected/provided to use for searching.
Having good quality metdata is what makes a website, database, or search engine great and user friendly. If your metadata is not comprehensive, then your users will not be able to find what they are looking for and will not return to your site.
Being able to enter multiple agents, elements, and attributes will help narrow down a user search to a manageable and useful amount of results and allow them to continue searching using the various metadata provided.