Neo,
DDE is basically a protocol that lets two different applications talk to each other. So you could have a program that talks to the TWS and sends in orders for you. Pretty much the only reason to do that is to automate order entry, either partially (you still make the trading decisions but automate the order entry to make it faster, for example by having a program determine the size, limit, executing a basket of stocks you've chosen, etc.,) or completely (have a program trade for you without any human intervention.)
voodoo