The M3A1 is United States' standard transport halftrack, shared with Great Britain and the USSR as the M5 variant.
Halftracks have very similar roles to trucks in that they provide both transportation and logistics support. Halftracks however are designed with light armour to protect against small-arms fire and do not need to unload their cargo; they automatically provide nearby infantry with logistics support, although in a smaller radius than a deployed truck. To top it off, halftracks are armed with a single machinegun to provide additional fire support for infantry -- in the M3A1's case, an M2 Browning (.50 caliber).
The M3A1 is a very useful support unit, as it provides cover and suppression fire for your infantry, and it's small supply radius makes it the perfect piece to complete an armored assault.
You really can't regret spending some command to produce this very versatile unit.