"If you delete a previous incremental backup, you will not be able to restore a later incremental backup unless you have another full backup right before that next incremental backup, or a differential backup right before to that incremental backup.
For example, we'll say you did a full backup 1 week ago. Each day after, you did an incremental. If you deleted the second day's incremental backup, you will not be able to restore the files properly even with the next 6 incremental backups.
successive incremental backups only contain information that has changed since the previous backup. If you want to get rid of a bunch of incremental backups lying around, preform a differential backup instead. The file size will be bigger, but you won't need as many files when you want to restore.
Full Backup: Everything on the system is backed up. The most amount of space is required to perform an incremental backup.
Differential Backup: Everything on the system which has changed since the last full backup will be backed up. Less space is required than a full backup, but it will use more than a daily incremental backup.
Incremental Backup: Everything that has changed since the last backup (of any type) will be backed up. The least amount of space is required to perform an incremental backup."