With regards to locking knives: most people would say that a locking blade is 'safer' to use and more capable of stabbing someone than say, a Swiss army knife, which would fold on your hand if you tried stabbing with it. But in regards to the actual laws that countries write and enforce, there's usually not even a reason listed for such a prohibition in places that have such bans.

For Germany, section 42A of the Weapons Act applies. That basically states (official english translation):

It shall be prohibited to carry (...) knives with a blade which can be fixed with one hand (one-hand knives) or fixed knives with a blade length of over 12 cm.

The original German for 'blade which can be fixed' (feststellbarer Klinge) is what we would refer to as a locking blade. It doesn't even attempt to give a reason as to why.

In the UK, where a knife with a locking mechanism is illegal as well, the Lancashire Police says only this:

A lock knife is not a folding pocket knife and therefore it is an offence to carry around such a knife regardless of the length of the blade, if you do not have good reason. A lock knife has blades that can be locked and refolded only by pressing a button. A lock knife has a mechanism which locks the blade in position when fully extended, the blade cannot be closed without that mechanism being released. A lock knife is not an offensive weapon per se, as these knives were made with a specific purpose in mind were not intended as a weapon. However, possession of a lock knife in a public place without good reason is an offence.

So this has a lot of contradictions in it. That first sentence makes no sense: 'a lock knife is not a folding pocket knife', when clearly there are knives, with locks, that fold. 'A lock knife has blades that can be locked and refolded only by pressing a button'. OK, so a button lock is illegal. Which means a liner lock is OK, right? But no. 'A lock knife has a mechanism which locks the blade in position when fully extended'. So now we've abandoned that button, and have moved on to mechanism... And then we get some form of argument as to why this all is banned: 'A lock knife is not an offensive weapon per se, as these knives were made with a specific purpose in mind were not intended as a weapon. However, possession of a lock knife in a public place without good reason is an offence.'

So there's a 'not offensive weapon per se', but also 'posession with no good reason is an offence'

Basically, the only thing you can reasonably have on you is a non-locking small Swiss army knife. Anything that even hints at a lock? That's a crime. Why? Fuck you, because we say so.