"View and manage your spreadsheets in Google Drive": This is necessary to read and write in your spreadsheet, for instance, to read first names, last names, and websites to return email addresses in a fourth column.

"View and manage data associated with the application": We need this permission to keep you logged in to your Hunter account directly in Google Sheets.

"Connect to an external service": We connect to Hunter's API to find email addresses and then display them in your Google Sheets.