Excerpt from "'Should I Buy It?' - Red Death & Other Exotics" (Mar. 13th, 2015)
An Insurmountable Skullfort is one of four Titan helmets; the other Striker helmet is Helm of Inmost Light. Skullfort's main feature gives you the Transfusion class talent, which regenerates health on Storm Fist/Shoulder Charge kills. It also spawns you with melee energy, and lowers your grenade cooldown when you deal melee damage. Another benefit of Skullfort is that it frees you up for the talent Headstrong or Aftershocks.
While Skullfort has some use, there are simply better choices in both PVE and PVP. (e.g., Crest of Alpha Lupi are great for PVE, etc.)
Its primary PVE use among the community is against Crota in Crota's End (Hard Mode). Other Guardians use their Special during the first phase, generate Orbs, then you can return to the room when you need to regen health (Infusion).