Yups, it's what allows you to install channels and wads into the Wii. There are other exploits like the ones that Twiizers and Comex have that allow BootMii and the HBC to install even on 4.0 systems, but the trucha bug is the only one that's public. Therefore we have to come up with tricks to reintroduce the trucha bug into the system to start installing cIOS and all that. That's why we use tricks like downgrading IOS35 and cboot2 to get the trucha bug on Wii's with updated firmware.