... when there is more than one child in the family. I am not advocating leaving the youngest member wistfully poking their nose through the bars. Instead, a playpen can be a perfect place for an older child to play without having their toys commandeered every five seconds.
Of course, every family should own a sturdy and large dining-type table. On the table is the ideal place for an older child to play - not at but on, feet and all. All chairs should obviously be removed so that the younger sibling can crawl/todddle around on hir seek-and-destroy lego mission without causing open warfare by attempting to scale the heights. The older child being out of direct view also helps the younger child fully to enjoy exploring the vast swathes of available carpet.