Sometimes you dont have access to your shell, maybe because the services of your hoster only includes ftp or whatever the reason is. But you want to set up a clean installation environment for your brand new typo3 website and you dont want to upload every single file from your compressed typo3 zip file. You can unzip files with the PHP function system. Just upload your compressed typo3 zip file to your ftp directory, create a php file which calls unzip, also upload this file to your ftp directory and with your browser you can execute this php file and your zip file should be uncompressed in seconds.
Now you can perform a clean typo3 installation, unless you dont get problems with other php settings during your typo3 installation check.