Set a group that sends out customized messages on birthdays, holidays, or even on anniversaries.
Groups allow you to segregate your contacts lists into more effective marketing demographics. Since you can create an unlimited amount of groups you can use an unlimited amount of variables in which to store your contacts.
... and many more
Essentially any metric that you can think of to separate marketing lists into, groups would be the most effective way to organize and store those contacts based on their different categories.
Another great function of groups is that it allows Marketing Companies who manage multiple clients' campaigns from one account to organize and separate their clients' text message marketing campaigns.