Using the connectionless subnets or the connect-oriented subnet is the 'To be or not to be question' in networking.
An analog (Tanenbaum uses this word frequently) is that I want to tell my girl friend how much I love her. One way I can just call to say I love you.(name of a famous song) The other way is write several love letters and send them to her. In first situation, the conversation is continuous. My every word will arrive in sequence. But there is always a annoying time before the connection is built.
In second situation, the post office is the key. They are in charge of sending every letter to my girl friends. Suppose the letters will arrive in deferent ways(shipping, air or the pidgin). In result, the word will in random order. Anyway she still gets the meaning as long as there's no letter missing in the transmission.
ATM (the abbreviation of Asynchronous Transfer Mode) is connection-oriented though it uses the packet switching subnet. Because it requires to build a temporary connection, and there is always a virtual circuit in the subnet. When the connection is set up, the data is transmitted in packages.