The idea behind Salt environments is to allow formulas to be distributed to multiple types of minions while still restricting them from running in areas where they are not appropriate. For instance, if you use LDAP as an authentication mechanism, it probably makes sense to have it available in every environment. However, even if you're using Git for revision control, you probably only want it in your development environment, as other environments are often deployed via some other mechanism.
By default, all minions will behave as if they are in the base environment, even if you have no environments defined. By convention, most Salt deployments make use of the base environment anyway, so it's not a bad idea to follow suit.
The base environment is designed to provide formulas that will be useful or necessary across all minions. Decide carefully which formulas to add to base: most formulas will have at least one environment that they do...