Just general calendar arithmetic. There are tons of cases where you need to do stuff like calculate "one month before", "one month after", "beginning of the year", ... --that's a mess with unix timestamps, but trivial with a good calendar library (or postgres, for that matter).
Well, yes, you can convert on input and/or output from/to unix timestamps, sure (and you can with postgres, too), but the point is that you have to convert first, you can't do date arithmetic on unix timestamps directly (because there is no fixed relation between unix timestamps and days).
