There is a small article on Salesforce help
You have access to the Sharing button when your sharing model is either Private or Public Read Only for a type of record or related record. For example, the Sharing button may appear on an account even though your organization-wide default for accounts is Public Read/Write, if your organization-wide default for related opportunities is Public Read Only.
When the button is available on the page layout, administrators see
the button on all records. Non-administrators see this button on
records that only they own.
Here you have an example of other scenario and an answer to your question.