MikeJones to give a brief explanation, flarum comes shipped with a built in searching system. Frankly, it's not the best, but it's light weight and it gets the job done without additional dependencies. Also, we're in the forum market, not the search market.
Especially for larger sites, it would be preferable to support custom search drivers that can solve some of the issues with Flarum's current system (including speed, foreign language search, search accuracy, etc, etc, etc). Hence, we want to make the search system extensible so that people can use those search drivers.
As to the differences between individual third party drivers, I don't really know tbh,that's something I'd consult other online resources for
