The fan isn't required when using PoE (or the Pi3+ generally) and will be software controlled (you can leave it turned off if you don't have thermal issues).
It was included partly as the PoE HAT has the space, and PoE is often used with racks of Pi boards which may be more likely to generate heat.
In normal usage Pi3+ heat dissipation is much better than a Pi3, so a fan isn't essential.
[^] # Re: POE
Posté par Julien L. . En réponse au journal Sortie de raspberry pi 3B+. Évalué à 2.
D'après dom sur le forum RPi, non: