I'd say its typically used as a status symbol, I use IRC a lot and frequent a lot of the same channels with "regulars" they all want to be voiced and OPed, but with no real reason other than its a position of higher status or power.
It give them a greater feeling of purpose and perceived respect.
If your aiming to get a position at an op, I'd suggest you just lie like everyone else, no one really wants to "help others" but its probably one of the most commonly used reasons.
I guess it depends more on what type of irc channel this is about, if you have been there more than a week you should already know what the OPs do, or at least are suppose to do, just tell them what they want to hear.
You shouldn’t have to ask on something like this, if you looking to apply, you should already have a motive, you just need to decide if it’s a good reason that will get you accepted and modify accordingly if its not.