page_get_cache sets all pages as temporary

Why does page_set_cache use CACHE_TEMPORARY for the expiry time? I simply don't understand. I've searched and cant find an answer... Maybe I'm using the wrong terms.

CACHE_TEMPORARY means no pages get refreshed based on the cache_lifetime setting. It requires the Cron job to wipe them. This means if you run your cron daily, a page will be cached for a day. If you run your cron hourly (say to keep your search index fresh) then every hour your cache_page table gets wiped.

Surely, if you set your cache_liftime variable to, say, 15 minutes that means you want your pages to only cache for 15 mins...

Have I missed something?