It’s unclear (and maybe someone can clarify) the relationship between the Stormfather and Honor (the Shard). Highstorms predate Honor’s arrival on Roshial. There were some Spren around before Honor arrived. Bits of Investiture left by Adonalsium that gained sentience. Syl identified the Stormfather as her father.
Spren are manifestations..sentient beings that represent natural processes (flamespren, starspren, windspren) or feelings (awespren, painspren). Perceptions (of themselves and others) affects Spren. Highstorms are a dominating facet of existence on Roshial. Makes me think Stormfather was one of the first Spren to gain sentience, and has significant power on his own. There’s some connection between windspren and Honorspren, according to Syl.
Maybe when Honor arrived, he sunk his own Investiture into the highstorms, further empowering the Stormfather and creating a symbiosis between them. As one of the oldest and strongest Spren, and one strongly connected to his Investiture, seems logical that Honor would turn to the Stormfather as he died, giving him the visions to pass on. Since much of the world sees the Stormfather as a manifestation of the Almighty, seems logical that belief would reinforce/buttress the Stormfather’s power and identity. Because people believe he is the Almighty, he becomes the Almighty, as much as is possible.