It's a fuckload of money for this guy, but this is also a bit of an early career Wenger-y signing, to take a big punt on a talented misfit who's not been playing in his best position, such as a certain Thierry Henry.
Our most recent similar signing was probably Ødegaard. I would guess Arteta and Edu have seen what Havertz can do in great detail, and have reasoned that form (and tactics) are temporary, but class is permanent.
Havertz can play in a few positions, and yeah he might play as our left 8 or not, but ideally we need him, like Trossard, to put meaningful pressure on Jesus, Saka and Martinelli to hold down their places in the XI. Like Jesus and Zinchenko before him, he will join the squad with a desire to polish his reputation in England—and as with Jesus and Zinchenko, that motivation is probably only his engine for six months. Let's hope that's enough to establish him as a great option somewhere in our attack.