Build-in debug mode in PrestaShop is a very useful tool. But it is not recommended to have it turned on in production environemt because it can leak sensitive information about your website to ordinary visitors.
This free module is here to help you with this shortcoming. It allows you to enable debug mode only for specific users / sessions. Module creates secret url, and anyone who access your site via this url will browse with debug mode enabled.
This way, you (or anyone you share this url with) can have access to debugging information, yet your normal visitors will not be affected.