confusion in use of exit 0

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http://www.unix.com – hi i am new to shell scripting. i was going thru the part option and arguments. on this section i fail to understand the use of exit 0 in below example . Code: #!/bin/sh USAGE="Usage: $0 [-c|-t] [file|directory]" case "$1" in -t) TARGS="-tvf $2" ;; -c) TARGS="-cvf $2.tar $2" ;; *) echo "$USAGE" exit 0 ;; esac tar $TARGS also, i got and when i use option c in the script. however directory created successful. below command i use to create the dir . Code: ./pmeter -c a3 and below is the error which i got Code: tar: a3: Cannot (HowTos)