Hello Natalia,
Logical Connectors will help you to group a set of connectors.
Using Connector Groups for ARA Rulesets, etc make it easier to migrate changes to Production environment as they are built against the group instead of the specific system.
When the connectors are created using Sm59, you are building connector for a specific system whereas logical connectors will help you to group these specific connectors into a group.
In Short: Connector group is used to refer to the component which can have multiple connectors. Specific Connector when you want that specific system.
Regards,
Deepak M