Applications that are waiting for anyone to mail them (i.e. server apps) need to "bind" a port and listen for traffic.

They generally do this on a specific port, this is so applications that want to talk to them can find them.

Applications that just want to start a chat with someone (i.e. client apps) can use a random one of the temporary ports as a return address for the duration of the correspondence.

If they use a port outside that range, then potentially they might prevent one of the applications that needs to listen to a specific port from starting (only one app can use a port at once). Likewise, if you decide to use a specific port in the temporary range, you run the risk it's randomly in use by someone else