Random move by Juventus I would say. Lemina didn't play that much last season, he was a backup to Imbula and Romao, before the latter was shifted to a CB position. He's a defensive midfielder, very good tackler of the ball, is quite disciplined and very energetic. He hasn't done too badly for L'OM, he's always been a very solid option from the bench. Last season he even filled in as right wingback in Bielsa's 3-5-2, when Marseille had an injury crisis.
He's a young player and still has room for improvement, I wouldn't be surprised if he turns out to be a very good defensive midfielder, as he's already exhibited that, but IMO he hasn't fulfilled his potential at Marseille. Back when he was at Lorient, he looked very promising at a young age, at the time Christian Gourcuff, one of the best managers in the Ligue IMO, and a manager who has been renowned for giving youngsters opportunities and developing them was Lorient's coach. I remember there was some controversy when Lorient's management decided to sell Lemina to Marseille at the end of the mercato a few years ago, apparently Gourcuff rated him very highly and thought that once Lemina got his chance as a starter he'd have been worth a lot more.
Still, there is a possibility that Juve have a strong scouting presence in France, and maybe Juve's scouts thought Marseille hadn't given him enough opportunities. He looked very promising at Lorient as a young defensive midfielder that was an excellent tackler, very snappy and energetic kind of player. But like I said, he didn't really get much opportunities at Marseille, hence why I thought this was a kind of random move.