Here is a quote from this guide.
https://www.coh2.org/guides/59348/usf-combined-arms-keep-what-you-can-secure#2492
The M1 75mm Pack Howitzer is the most powerful non-doctrinal indirect fire weapon team. Its auto-attacks are devastating and its barrages can lay waste at very long range. The Howie’s auto attack matches that of mortars at a range of 80, while its barrage has a staggering range of 160. In comparison, the OKW LeiG has a range of 100. At vet 2, its barrage is replaced with HE rounds, greatly increasing its barrage damage output. HE rounds also deal a decent amount of damage to vehicles and tanks. Note: even though it has a crew of six, it will be decrewed if the team is reduced to less than three members.
Two Packs are very good but expensive. They will heavily bleed your opponent’s manpower. However, if you obtain them without proper support, you will lack reliable capping force and thus your enemy will outcap you, so he can obtain tanks more quickly. Go for a second Pack Howitzer, only if you can afford it without hurting your field presence, or when your enemy goes for a support weapon team heavy strategy.
This unit can be used both offensively and defensibly AT THE SAME TIME. Place the Howitzer near your forces, so it can help them deliver a final punch, force HMGs and PaKs to fall back and possibly even wipe clumped squads. Always reverse your Howitzer when the enemy attacks hard. so you do not lose it.
It is important to place the gun defensively! Target priorities are:
Heavy Machine Guns
Anti-Tank Guns
Groups of infantry
Static vehicles
Single infantry squads