[SalesForce] Sobject Constructor and ID

This is a named parameter language feature that is only supported for specific SObject types and nowhere else in Apex. It makes code where you want to assign several values cleaner - see this example. Agreed that the documentation isn't good on this.

If you want to initialise from a map, you have to write your own utility method such as:

public void someMethod() {
    ...
    Account a = (Account) init(new Account(), new Map<SObjectType, Object> {
            Account.Name => 'Acme',
            Account.BillingCity => 'San Francisco'
            });
    ...
}

private SObject init(SObject sob, Map<SObjectField, Object> values) {
    for (SObjectField field : values.keySet()) {
        sob.put(field, values.get(field));
    }
    return a;
}

that makes use of SObject.put(SobjectType, Object).

You can also use strings as the keys (though you lose the compile-time checking that the field names are valid) as there is also SObject.put(String, Object).

If you want to write a utility that creates records, this function is absolutely necessary in order to be generic at all. Sure, you could also use Type.forName, but that doesn't take nearly as good advantage of strict typing.

Simply contrast these test utility styles and imagine which will balloon massively each time you want to support new object types:

public static List<SObject> createRecords(SObjectType sObjectType, Integer count)
{
    List<SObject> records = new List<SObject>();
    for (Integer i = 0; i < count; i++)
        records.add(sObjectType.newSObject());
    return records;
}

public static List<Account> createAccounts(Integer count)
{
    List<Account> records = new List<Account>();
    for (Integer i = 0; i < count; i++)
        records.add(new Account());
    return records;
}

Also, note that the optional parameters allow you to instantiate the object with its default values, a useful feature that you would not be able to accomplish using your first approach (concrete type instantiation). This functionality is quite useful in writing tests that perform consistently as expected.