This guide complains what's the purpose of the shrine of secrets.
Allows players to purchase teachable perks that're unlocked straight away upon purchasing.
Mostly used for DLC character perks.
For example Ashes mettle of man perk is available at the time of writing.
Can also buy pre owned perks for blood points.
In the store page with all the cosmetics.
Teachable perks are just perks that come with a certain character.
Each perk can be unlocked for all other survivors when they hit level:
Resets every Wednesday and shows four new perks that havn't been in the shrine for three weeks prior.
The shrine of secrets takes Iridescent shards which can only be earned from leveling up in online ranked matches, excluding kill your friends.
Average perk from the shrine is 2000 shards.
DLC perks can be 2,700+ shards.
You can buy perks that you already own in the shrine for a payout of 150,000 blood points.
In most cases it's not worth it at all.
Don't be lazy and actually level your characters up to the 40 rank.
You can get nearly all perks without needing to use the shrine.
When should you use it?
When you are 100% certain that a DLC teachable perk is available and that you won't ever buy that character. This will allow you to use the perk without the DLC.