I know WHAT trench warfare is. I've read/seen a lot about HOW tench warfare was executed. But I can't find a satisfactory answer for WHY trench warfare was used.

Neither side gains or loses a lot of ground using this technique. So what is the point?

It ends up creating a no-man's-land. But why is this no-man's-land different from other contentious borders, for example, the DMZ between the Koreas? Why in WWI did trench fighters see actual fighting instead of just creating a stand-off between sides?

I'm assuming it has something to do with the invention of tanks and larger explosives. ¯\(ツ)/¯