Is the dealer a volunteer in Shamir secret sharing scheme?
If no then what is the purpose for him to involve in the scheme?

This sort of &quot;why would someone perform this protocol&quot; question is something that crypto does not address.
Perhaps the dealer is the author of his own will, and he is delivering shares of it to his inheritors (for them to recombine after his untimely death).  Perhaps he is the owner of a cryptocurrency purse, and is distributing shares of his private key to save as backups (so that, even if one of his backups is stolen or lost, he's still safe).  Perhaps he is an HSM, and is distributing shares to the key that unlocks the administrator account.
To the protocol, the &quot;why&quot; question doesn't really matter.