IOS55 is an IOS that only three games appear to use -- "Need for Speed: Undercover", "Sonic and the Black Knight" and "The Conduit".
This IOS is presumably required by Undercover to make a Logitech USB steering wheel work. However, this does not explain why the other games require it although, in the case of The Conduit, it is possible that there is a connection between its usage and support for the Wii Speak periphal (it has been reported that both IOS38 and IOS55 are required by the official Nintendo Wii Speak channel).
Typically, if a game that requires this IOS is started without it (via brickblocker or a Starfall update skip, for example), then the screen will turn blue and a white-text error message "Error #002" will appear. The Conduit behaves somewhat differently as it will simply display a black screen before freezing.
Installation from the Undercover disc keeps System Menu 3.3 and does not(!) install System Menu 3.4, assuming the user's Wii didn't already have 3.4. On the other hand, The Conduit appears to contain a complete System Menu 3.4 update.
When launching Donkey Kong Jungle Beat(Wii) and Excite Bots with Gecko OS it says IOS55 required when on System Menu 3.2US.