Why Punch Through ?

Firstly, Punch Through does not effect this weapon like you would expect that an explosion type weapon would.

But rather, it behaves like a hit-scan weapon

[Shred] or Metal Augur, on hit scan weapons, almost always applies a DPS boost. They allow you to shoot through dead targets and light/medium cover. This alone can be a big boost. Then, you need to account for being able to hit several targets per shot and/or inflict multiple status procs per shot. An action that is impossible to avoid with the [Trumna]. This also causes you to become much more ammo efficient witch is very important at higher levels of play when everything is a bullet sponge.

Punch Through is removed on the [Trumna] Alt Fire projectile.
The main feature of the Alt Fire is the radial attack on each bounce, and Punch Through can make it disappear through geometry. Disabling on the projectile only allows Punch Through to be used with the Primary Fire without affecting the Alt Fire.

Here is just a little math to give you an idea of how good it can be

Lets say you do 100 damage per shot..

A 90% damage mod would add 90 damage. Each one after that gives a lower overall bonus as percentage as its based off that 100 damage. The total damage increase as a percentage is something like 90%, 47%, 32%, 24% per 90% elemental mod per shoot in succession. Punch Through causes 100% per target hit after the first. So, hit two deals 100% more total damage from that shot, and hit three deals 200% more this is because you deal full damage to each target, and so on. When I say "hits" it refers not to separate times the gun is fired but rather each projectile of that shot and the number of times is does any sort of damage proc.
If you overkill a target with extra shots or the AoE effect of Trmma, it still deals damage to the enemie(s) behind.

In addition to that, [Shred] still gets almost the same fire rate (55% instead of 60%) you would get with [Vile Acceleration] (a popular choice) without the negative 15% damage.

In many weapons even all that isn't worth the mod slot.

But...

Because of how to Trumna shoots, and it being treated like a hit-scan weapon and not a launcher weapon, it is one of few exceptions to that!