Many restaurants give guests electronic coaster pagers that buzz and light up when their table is ready. The devices aren't cheap, but they do have an important role of somewhat "locking in" patrons to that particular restaurant.
If the hostess writes down your name and you get sick of waiting, you can discreetly walk out without saying a word and go in search of somewhere less crowded.
Instead, if the hostess gives you one of these electronic pagers, your only ways out are awkwardly handing the pager back to the hostess and telling her to take you off the list, walking off and basically stealing the pager, or just waiting longer than you wanted to for your table.
The hassle associated with the pager of course won't stop everyone from leaving, but it surely stops quite a few. And that can make a big difference at the end of the night for a busy restaurant.