From MSDN Back Up Database Task (Maintenance Plan)
Backup set will expire
Specify when the backup set can be overwritten by another backup set.
So, there's no deleting of old backups, just overwriting with new ones.
Any advice on how to use "Backup set will Expire:" options while creating Maintenance plan will be appreciated.
This depends on how long you want to keep your old backups.
Does your company have a policy for that?
Do you move your backups to a storage after a while? If you do, you can specify any number of days for expiry, as long as you copy the backups before the expiry time comes.