Why is this better than that?

Why does something being hand made make it better?

Sounds good. People see value in the idea.

I hear much about something called '531' apart from being lighter (is it?) what difference does it have to my cromo 4130 (or something like that).

Even if it is lighter it is rarely by a significant amount.

Is NJS like Corgi? My bike has lasted 12 years of of daily riding and still works fine.

That's great, stick with it and here's hoping you get another 12.

Are we simply buying these names for image based reasons?

Beyond any tangible advantages, of course.

I am puzzled why anyone asks these kinds of rudimentary questions, people are attracted to stuff beyond something's utility.