Do you have any idea what MobileSubstrate does? It's a very sophisticated and extensible code injection system which allows tweaks to safely insert code into native applicators and into the operating system itself to modify how they'd run. It has numerous APIs that allow developers to do things with iOS that Apple never intended. And it does all this while providing you with a safe way to recover from crashes and uninstall tweak that would otherwise require you to restore your phone.
Tell em bro