It is very useful (and valuable security-wise) to be able to reverify hashes and signatures "on-engine" instead of dragging material out to a client app and running the algo there. The implementations are straightforward and everywhere now so it's not a huge lift for the backend. Example use:
aggregate([
{$match: whatever},
{$addFields: {
hashok: {$cond: [ {$eq: [ {$sha3: "path.to.struct"}, "path.to.stored.sha3"} ], 1, 0},
sigok: { $verify: { sig: "path.to.sig", pubkey: "path.to.pubkey", algo: "name of curve to use eg. SECP256k1}}
}
])
The digest function would operate on the raw BSON behind the scenes.