The purpose of this build is to be compatible with BossMan's Smeeta build. This is the mod placement for the tank non-steel essence farming use.
[Synth Fiber]:
It allows you to pickup Health Orbs (and proc [Equilibrium], the warframe mod, to give you energy at 27 per health orb) even if your frames health is max.
Additionally the armor bonus is applied after Link Armor, so you get a lot of armor (especially with frames that have high base/increase it)
Mediocre Alternatives:
1) You can replace Synth Fiber with Hastened Deflection (shield gating) or Loyal Companion (bleedout) if you forma [Medi-Pet Kit]'s slot. But [Synth Fiber] is better imo.
2) [Pack Leader] can be changed for [Hunter Recovery], but your Smeeta won't heal you much. So it's not worth it. If you want your companion to heal you, use the [Panzer Vulpaphyla] build from @SatireVer