Explain why an organization's firewall should block outgoing packets the source addresses of which are not in the organization's address space.

A firewall is used to monitor and control incoming and outgoing (or in better terms, inbound and outbound) traffic in a network based on some pre-defined  security rules set by either an individual or a business.

Now, using firewall to block outgoing packets that have source addresses which are not in an organization's address space means rejecting requests that, though originate from the organization's address space, are not in the organization's allowable address.

This is necessary and of utmost importance because even though an hacker (or simply an attacker) gains access into the organization's network, there is a limit to what harm they can do. If for example, they manage to get some malicious programs into the system's via network processes such as using infected e-mails or using a browser, they might also want to make some requests to the Internet to get some additional programs downloaded which altogether is harmful to the system. Disabling or blocking outgoing packets or traffic will stop this combined hazard from happening.

well you could find somebody who doesn't want theirs and ask for it or possibly trick a dealer into giving it to you for free.