So, there is a lot I could tell you about Guardians and Destiny and the Light so I'll just...
Grimoire: Guardians (also has info specifically on Warlocks, Grimoire Cards were offical lore released within Destiny 1.) Guardians (general wiki page) Guardians (Destinypedia, a little more in-depth) Ikora herself
But, short bit: Guardians are resurrected from the ancient dead because they're capable of wielding the Traveler's Light. Their Ghosts can resurrect them from their dead, is a general advisor and companion. The Light gives them access to their powers.
no subject
Grimoire: Guardians (also has info specifically on Warlocks, Grimoire Cards were offical lore released within Destiny 1.)
Guardians (general wiki page)
Guardians (Destinypedia, a little more in-depth)
Ikora herself
But, short bit: Guardians are resurrected from the ancient dead because they're capable of wielding the Traveler's Light. Their Ghosts can resurrect them from their dead, is a general advisor and companion. The Light gives them access to their powers.