The global_merge_vars parameter allows you to pass global values for a particular merge tag, to be applied for all recipients in that API call by default, while merge_vars allows you to specify per-recipient values based on the recipient email address (this is why the rcpt is declared for merge_vars). The content in the email or template doesn’t vary – these API parameters are just distinct since they apply differently.
The template_content parameter is for editable regions in the template. More details here. Originally, Mandrill didn’t support merge tags, and only supported editable regions in templates. For backwards-compatibility, template_content is still required when calling a template. It also helps to ensure that even if a template region isn’t to be replaced, that’s done explicitly by passing a blank array.
The Mandrill KB has quite a bit of info on various topics, and you can submit a support request there if you have questions specific to your account, or in general!