Well he's playing there, but I don't think we bought Asamoah to make him our dedicated LB/LWB.
We needed a good CM because we didn't really have a solid senior player for rotation (I'm not a fan of Giacch in the middle, too lightweight, Marrone still unproven, Pogba hadn't played any minutes in Serie A), so we bought one of the best young box-to-box CMs going. Asamoah is only 23.
I HIGHLY doubt that he was bought to play on the left, but DC was hurt early in the season, and we don't have great options. Seems like we almost bought Peluso in the summer, were interested in Armero, and one of them would have rotated with DC.
But Armero was too pricey (not surprising that Udinese didn't want to sell us 3 of their better players), and I'm assuming Marotta didn't buy Peluso because we have enough mediocre players already (even if that hasn't stopped Beppe at other times

)
Asamoah is playing there, imo, because he's better than the other options, and it makes sense when you buy a new player who you envision being a member of the squad for a long time, to get him into games, get him settled into the team ASAP.