i remember they were described somewhere, if only i could recall.Anyway, they already sound useless to me

I believe they're somewhat described in the demo contents.

I'm kinda thinking each one is for what you might call a "sticky" but not exactly. Like, an intro-cat post shows up when a page is category-based. After an absolutely miserable time dealing with 310's "you must have these avatars that suck" feature I didn't play enough to find out if there was even the slightest control over who gets to post intro-whatevers or what happens when someone creates a second intro-whatever.

Intro-Cat is a post that will show as a intro text on a category.
Intro-Tag is a post that will show as a intro text on a tag page.
Can't remember Intro-All,Intro-Main, Intro-Sub.

Podcast is a .. podcast ;) Just put a link to your mp3 file on the link section, and b2evolution will show a player on your blog ;)

Intro-All will show to all Categories regardless of level.
Intro-Main will show only in all Top-Level Cats.
Intro-Sub will show only in all Sub-Level Cats (ie. Categories below Top-Level).

From what I understood ;) I haven't tested much with the new post types, but I do see some use for it.

I updated the manual with the info I could found.
Please correct if I was mistaken (or correct my english)
http://manual.b2evolution.net/Write_tab#Post_contents