I learnt "on the job", so to speak
The argument for a blade designed for stabbing is more in the tip than in the actual length. The normal reason for a long blade is precisely the length go the cutting edge. As an example,
Please remember that at all time I consider a knife as a tool (those classified as weapons per se are mostly illegal to even own in the UK, although there are exceptions like antiques (say, sword canes) or sports (kenjutsu))
The best example is of course the kitchen, where you have loads of different knives, of different blade size and blade shape.
It is however perfectly possible to use a tool as a weapon. In which case even your keys can get you into trouble.
In the case you are suggesting the premise is you have a blade longer than 3", which imply you have a good reason to start with (otherwise carrying it is illegal regardless of whether you use it or not).
Now an example is me taking my
Leuku to clear the footpaths when the brambles are starting to obstruct them.