BINDing directories is similar to creating another Hard Link. I find it useful when say for example, CIFS or Deluge has issues getting too far up the file tree due to permissions. I will create a directory under /srv/samba and bind the high directories. Certain programs see symlinks *soft links* and won't act correctly that is where Bind comes into play!
You can use it in FSTAB and also manually via CLI
FSTAB:
/olddir /newdir none bind
Manual Mount
mount --bind /path/to/olddir /path/to/newdir mount -B olddir newdir