In the advanced section of the approvals template settings, you will see a checkbox called "Further approval needed in sequence".
This checkbox has 2 functions.
1. Checking this box prevents the 'approval granted date' from being set on the ticket by this template under any conditions (even if the approval is granted). The reason why you might want to do this is that you have other approval templates that will take place after this one.
For example in the image above, when this approval is granted, it's been set to add the tag "travel_approved_manager" to the ticket. This tag is what could then be used to initiate further approvals that need to take place before the ticket is properly deemed to be approved. See here for more tags and actions on tickets.
2. When you check this box it ALSO allows the approvers to add a comment when they grant the ticket like this:
Normally, if the approval is the last in the sequence - or the ONLY approval needed, the approver would not be able to add a comment, since the approval is already granted - what they say would have no further bearing on the approval.
However, if there are other approvals to follow, the first approver may want to pass information onto the second approver which may have bearing on the success of the secondary approval.