0.5/0.38 a very fine line? I'm using Staedtler pigment liners which are 0.05. The downside is that my handwriting has become positively tiny...

0.5 is pushing it, but for normal size writing 0.38 is a sweet spot of fine for most people. I use a 0.3, and I immediately notice bad paper because it becomes somewhat scratchy. This is in gel pens.

Looking at jet pens, yours seems to be a marker pen, which generally needs to be thinner so it won't bleed. Similar to how ball points need to be fat (0.7mm is called fine) to write smoothly and make a dark enough line.

Why specifically this brand? Is it special, and is the difference between an avg random pen big?

Frixion pens are erasable.

The ink turns invisible when heated. The pen has a rubbery tip on top which acts as an eraser when rubbed on the ink.

You'd be surprised. Pilot G2 is a great writing pen, and I'm personally a fan of Uniballs.