My apologies for first making a detour from the OP's original question, but I must protest whenever someone requires a justification for someone to carry or do something (the "why would you need..." question). The main issue is whether it is ok to say that someone else can or can not do or own something. To that point, "need" is simply the wrong standard because:
1. It implies a double-standard. There are many thinks that one could argue that are not "needed", at least in a strict sense: alcoholic drinks, tobacco, etc... We don't apply the "need" standard to those, so why should it be applied to other things?
2. The way I see it, the whole point of the constitution, and the bill of rights in particular, was to create a county where people were allowed to live their life peacefully while agreeing to disagree on many aspects. Therefore, unless someone's owning or doing something is per se a violation or infringement on someone rights, liberties, or general ability to live their life according to their terms, lets all please stay out of it and let each other live.
Unfortunately, there are many laws across the country that violate these principles. Frankly, the vast majority of politicians, across the political spectrum, seem to not abide by these ideas. Unless these things are, hopefully, corrected in the law or flushed out in the courts, it is prudent to be aware of what the laws out there are saying. Hence, the OP's question is entirely pertinent. Hence, even though I have ZERO interest in carrying an auto knife, I fully support the right of people to do so. Moreover, just to answer the "need" question nonetheless, I could see how closing a knife one-handed is hazardous; an auto avoids that hazard.