How this is updating contact's mailing address related to account's billing address?

trigger ContactMailAddr on Contact (before insert, before update) 
{
    set<id> setofaccountIds=new set<id>();
    
    for(Contact contact:trigger.new)
    {
        if(contact.AccountId!=null)
        {
            setofaccountIds.add(contact.AccountId);
        }
    }
    map<id,account> accountmap=new map<id,account>([select id,
                                                    billingCity,
                                                    billingState,
                                                    billingpostalcode,
                                                    billingStreet,
                                                    billingCountry from account where id in :setofaccountIds]);
    for(contact contact :trigger.new)
    {
            contact.MailingCity = accountmap.get(contact.AccountId).billingCity;
            contact.MailingState = accountmap.get(contact.AccountId).billingState;
            contact.MailingStreet = accountmap.get(contact.AccountId).billingStreet;
            contact.MailingCountry = accountmap.get(contact.AccountId).billingCountry;
            contact.MailingPostalCode = accountmap.get(contact.AccountId).billingpostalcode;
    }
}