You can subscribe to notifications on a ticket to keep you aware of changes, even if you're not the ticket owner or a collaborator. Use it when you want to know about what happens on a ticket even if you're not directly involved with the work.
You'll always receive a notification if you're mentioned or assigned a discussion thread, regardless of whether you've subscribed to notifications on the ticket, as long as you notifications enabled in your notification preferences.
You must enable notifications in your notification preferences to actually receive them. If not, subscribing to notifications on a ticket won't do anything.