why do we need execution permission to change a directory in Unix

why do we need execution permission to change a directory in Unix

Here are the permissions given on the machine:
drwxrwxrwx   4 root     root         512 May 16 09:32 STC_10
drwxrw-rw-   4 root     root         512 May  5 11:22 STC_11
Now here is the Problem:
cd STC_11
-bash: cd: STC_11: Permission denied
But this one works:
-bash-4.0$ cd STC_10
-bash-4.0$ ls
Codemgr_wsdata  src
Why do we need execution permission to do a reach that directory STC_11 ?? Wont read-write permissions suffice ?? EDIT: An ls command on STC_11 would work.