According to Paul Joyce of The Times, sporting director Michael Edwards "insisted" on the clause, which made Barcelona "unhappy." Joyce added the clause "has effectively warded off the threat of the Catalan club raiding Anfield in the short term."
As an example, Joyce noted that "should Barcelona try and sign Virgil van Dijk, for example, he would cost the surcharge as well as a fee which would far exceed the £75 million Liverpool paid for the Holland captain 10 months ago."
Coutinho made the switch to Barcelona in January after months of speculation leading up to the deal, and the Catalans made a poor impression in the process. Some of the club's players spoke openly about wanting to play with Coutinho before signing, including Ivan Rakitic in an interview with Sky Sports.
It marked the second time in just a few years the Spanish champions landed Liverpool's biggest star, with Luis Suarez making the switch after the 2014 FIFA World Cup.
According to the Mirror (h/t Anfield HQ), the two teams are set to battle in the transfer market again and are looking at Ajax starlet Matthijs de Ligt:
Ousmane Dembele has also been frequently been linked with a move to Anfield, per Sport (h/t Sport Witness):
Roberto Firmino is said to be a target for the Blaugrana, and last month, OK Diario's Eduardo Inda (h/t Express' Jack Otway) reported Barcelona were planning a move for the former Hoffenheim star in 2019's summer transfer window.
Such a transfer would be impossible if the clause is in place, as Firmino is a key player and just signed a new contract in April, suggesting he has no interest in leaving in the near future.