Sure, say you calculate 2 weeks from now, and you want to add a year. This is where you'll see the benefit of strtotime over mktime. With mktime, you'll have to split up your date.
/* get two weeks from now */
$date = date("Y-m-d", mktime(0,0,0, date("m"), date("d")+14, date("Y")));
/* now add the year */
// strtotime
$new = date("Y-m-d", strtotime($date." +1 year"));
//mktime
$split = explode("-", $date);
$new = date("Y-m-d", mktime(0,0,0, $split[1], $split[2], $split[0]+1));
Both produce 2005-07-05, but strtotime eliminates the need to split the date up beforehand.