Tell that to Klopp and Firminho.
You're the one saying he doesn't add anything, what doesn't he add that was there before he arrived?
Firmino DOES add quite a lot besides scoring, unlike Højlund.
For obvious reasons, it makes little sense to ask someone to list everything he doesn't add, as if it could otherwise automatically be assumed he makes great contributions in every area. But if you insist:
Shots: 1.78/90. 9th best on the team.
Completed passes/90: 12.8. Worst on team.
Progressive distance of passes/90: 43.1. Worst on team.
Key passes/90: 1.04. 13th on team.
Passes into final 1/3/90: 0.67. Worst on team.
Passes into the penalty area/90: 0.61. 15th on team.
Progressive passes/90: 1.53. Worst on team.
Shot-creating actions/90: 2.52. 11th on team.
Goal-creating actions/90: 0.18. 13th on team.
Tackles won/90: 0.25. 23rd on team.
Blocks/90: 0.18. worst on team.
Interceptions/90: 0.06. Worsted only by Martial and Hannibal, among players with at least 90 mins of playing time.
Clearances/90: 0.67. 19th on team.
Touches/90: 25.2. Worst on team.
Touches attacking 3rd/90: 12.9. Tied for 17th.
Attempted take-ons/90: 1.35. 14th.
Successful take-ons/90: 0.74. 11th.
Progressive carries/90: 1.35. 13th.
Carries into attacking 1/3/90: 0.80. 19th.
Carries in the PA/90: 0.55. 7th - but already Antony, who is right ahead of him on the list, has nearly three times that.
Team success +/-/90: 0.12 . 10th.
Loose balls recovered/90: 2.64. Worsted only by Martial.
Aerial duels won/90: 0.98. 13th.
What was or wasn't there before he arrived is neither here nor there - that says something about our other attackers, not about him. If you want to seriously argue he's making a big impact even without scoring, explain what that impact consists in.
He scores goals. That's the overwhelming part of his value. And there's nothing wrong with that.