thanks for asking. By default, Kentico uses the cookie (or sessions if the Remember contacts permanently setting is not used)to recognize contacts that access the site.
This is basically the best option, however, there are scenarios, where some of our clients use the extra settings for recognizing contacts based on the User agent and/or IP address if the contact is not recognized based on the cookie (for example another browser or another device).
My personal recommendation would be not use these extra options unless you really need to as it may introduce issues with contact merging if not used properly.
As for global contacts - if you only have one site or if you are using sites in your instance for different clients, it makes sense to turn this functionality off. On the other hand, if you have Blog, Ecommerce site and some microsite, where all of these belong to one company, I would recommend using this feature.