ByeByeGenerator was created primarily to remove the generator meta tag. That one is absolutely not necessary and can be a security risk by providing the exact joomla version to an attacker.
The robots tag is used by the search engines, if present. If the tag is not present, the search engine spiders (robots) assume they're allowed to index because there is no indication that they are not allowed to index.
If you want to keep the robots tag - then shut that function off. The plugin offers that as an option because it's a personal preference.