Issue Reminders plugin allows any user to create reminders for the issues easily. It has a lot of flexibility to determine how reminders are send and managed. Although you may use JIRA's subscription feature to send some kind of reminder emails you don't have many options to configure content and target of these emails. Also this is a power user type of operation and unreachable by most JIRA users. As you will see through out this user manual Issue Reminders add-on does a lot more than JIRA's subscription feature.