Key points to remember about manage access request in SharePoint:
1) From SharePoint UI, manage access request functionality is only available on site collection level.
2) If sub-site has broken permissions level, then we can set access requests email on sub-site level as well.
3) If we enable manage access requests on SharePoint site, then user can see "Request access" link when they try to access site in which they doesn't have access.
4) Manage access request functionality is very useful when we need to determine site owner.
5) By clicking "Request access" link, system will send an automated email to username who is listed in manage access request "Send all requests for access to the following e-mail address:" textbox.
PowerShell script for setting request email on SharePoint sub-sites: