-Able to erase off small mistakes
-Pencil lasts longer while ink can potentially fade or damage the paper
My 2 personal reasons. I don't have OCDs but it's the closest thing I can use to describe the way I write. I'm very particular about formatting, indenting, spacing and neatness. Sometimes I'll write a word and erase it like 5 or 6 times if I can't get the "y" or the "t" right. You can imagine how messy my pages would look if I had used a pen.
I used to use a water-based Parker IM and although I miss some of the upsides like the disgustingly smooth feeling of ink gliding across paper, I just can't deal with not being able to erase.
I also find comfort in knowing my writing will last till I'm dead, I won't have to worry about keeping my notebooks away from the sun and so forth.
The Tombow Mono HB, yeah. Seemed a bit too dark for a HB core but still really great writing experience. Felt like a 2B. Quite smudgy if you are particular about that