From Halopedia, the Halo wiki
The distinctive 'tuning-fork' shape of the Spirit comes from the twin carrier bays that extend out from the pilot's compartment at the rear of the ship. The Banished have reengineered the Spirit's contragravity system so that its chassis can take the strain of transporting prefabricated buildings to forward command centers.
Halo Wars 2
Like their counterparts in Halo Wars, Spirits are used to transport infantry and vehicles around the map. However, they now use their onboard plasma turrets to fire at enemies, and can also be called in during multiplayer matches. Spirits are also used to drop off socket base buildings, similarly to the D77-TC Pelican employed by their UNSC counterparts.
List of appearances