Why does the hemlok exist?

It's not like you get to choose all your weapons.  You fight with what you can find at first.

Since we're pretending you get whatever you want, sure, I tend to pick up other things.  First off, boosted loader is a near useless upgrade so I can tell you I never bother with that.  It's a matter of it's still pretty good at long range without dealing with the arcing fire of a 30-30 and heavy rounds in fact do more damage than light.  Yes, I too would rather use a 301 or Flatline, but there is a gun spam out there now and just like characters, the more of them there are, the clearer it will be which ones are better.

Still, you mostly chose your character.  You do not chose which guns you get to pick up unless you last.  And if I have a Hemlock and someone else has a worse gun, I've still got the odds in my favor.