That is something different, it is only used to waste bandwidth from someone (or potentially clogging server's upload, but its easy solvable), but in big DDoS attacks the attacker usually has several hundred thousands of zombies infected in his botnet, and then he orders all those zombies to spam packets at an IP he orders ... Every infected PC uses his maximum upload to target IP, resulting into something like this: http://d.pr/i/kmAn

If I'm online during the attack and check iptraf or tcpdump, I can see literally hundreds of different IPs spamming random stuff at me, completely overflowing my download until I get totally disconnected from server (time out), and I can do nothing about it, just watch it being offline ...