Re: It's funny people use messaging apps...
WhatsApp presents a better interface than (the default) email client on Android for exchanging a few quick messages back and forth. If you were to conduct a time and motion study, you'd find it a better tool for the job. You don't need click on 'expand all', for starters.
In addition, SnapChat notifcations result in a different noise and notification-LED colour to emails, so you know that it a message written for you, from someone you know well enough to have your telephone number, and probably relates to something that is happening soon. An incoming email could be from work, SPAM, marketing, or something else that doesn't need your attention.
The lock-in is small compared to some other services, since your friends' unique identifiers - their mobile telephone numbers - is independent of SnapChat. Also, people don't tend to go back over old messages, since it tends to be used for 'conversations' about what time to meet up that afternoon, i.e the messages are of no interest or value six months later.