Your login is normally your name on Google Workspace, for security reasons, but you may also be responsible for keeping up with emails to an address like sales@ or accounts@ (these are referred to as aliases, but could also be a group)

When you are part of a role based address like that, you will automatically receive those emails, but when you hit reply the sender will see your email address, not the sales@ or accounts@ address they sent it to. 

You might want to send it from the alias/group address because: 

• There are more people in the team that receive these emails
• You may job share with someone else
• When you move on to another role you don't want to keep receiving these emails
• The customer knows that alias email address for continuity, should only see that address

Here is how to configure your email to be able to send from the alias address you are already able to receive from