BootMii/boot2 is brick protection, the best there is. It lets you backup your Wii's internal memory, and restore it if your Wii stops working. Generally most bricks can be fixed like this (missing System Menu, missing System Menu IOS, region bricks, Error 003 (if bootmii is accessible) banner bricks, semi bricks etc.). It also allows you to launch applications using "MINI" which is Team Twiizers replacement for IOS. It basically allows full access to the Wii's hardware and allows for more powerful homebrew. However, not many developers have made apps for MINI yet. There's a Linux kernel and a NAND formatter and that's about it. I think the SDK may be harder to use than the regular one for normal homebrew.
Anyway, that's why BootMii/IOS exists, so that people with newer Wiis can use MINI apps. It has no other real benefit installed as IOS.