Address groups are very useful if you often send email to the same list of people — your family, a group of people at work, or people with which you are engaged in a club or hobby. It is a lot faster, and less error prone, to address an email with one address group than with a long list of individual addresses.
If your email program address book doesn’t have a group feature, then you can create a manual group manually by creating a text file, and then storing the email addresses in the text file separated by commas. You can then open the text file, copy the email addresses, and paste them into the To field of a new email whenever you need them. You can also put the list of email addresses in the body of an email that you then send to yourself, so that you can quickly access the addresses by opening that email without having to open an external text file with another program.
Sometimes you can conveniently make a temporary address group by using the “reply to all” function on an email that was sent to a group of people that you’ve already communicated with. You can then change the subject and replace the body of the email before sending a new email, thereby saving you the trouble of adding all of the addresses one by one.