Interesting little bit of stat that I stumbled upon on Transfermarkt.
Those are Wolves exclusive but NOT exclusively PL numbers (they involve all the competitions he played in with Wolves since signing for them):
As a Left Winger: Played 76, scored 19 and assisted 7
As a Center forward: Played 37, scored 18, assisted 8
As a Secondary striker: Played 12, scored 3, assisted 5
As a Right winger : Played 1, scored 0, assisted 0
It's a simple bit of statistic but it gives a good little insight into what we are getting but more importantly into what role Klopp and his staff are, presumably, eyeing for the lad. Based on those numbers I think he is brought in to be competing with Mane and Firmino. However, based on his qualities and how he seems to be a bit more comfortable in the middle (as his numbers are a bit more prolific there) I'd say he will be given a lot of games in the Firmino role.
The fact that he can do a good job in both positions also means that he will probably see a lot of games. With the insane schedule we're gonna have this season some heavier than usual rotation will be needed. This, along with the fact that he is proven PL quality, justifies the price tag more than what we would've paid for Sarr.
Both are quality but Sarr played 26 of his 28 games for Watford on the right wing which is not ideal considering we need flexibility and as much quality cover across the frontline as we can get and this would've ultimately translated into less games played by the lad which, again, does not help justify a heavy price tag (in the very basic terms of money/games played ratio).
I think that at this level of the game sometime you might need cover over the entirety of your frontline but the quality available paired with the money you can shell out ultimately dictates which position you end up covering.
There is also a point worth being made that if you need to rest Salah you can easily throw in Jota at LW and switch Mane over to RW and retain a mean attacking edge. You can also throw in Jota at RW if necessary because, if I remember correctly, the lad is also super good on both feet.