what's the actual function of the frameset?

Answer 1

<frame> and <frameset> are deprecated in Html5, and cannot be used... they were Html4 elements only allowed in a very specific flavour of Html4, requiring a specific <!DOCTYPE> declaration... They were used to build a page with many html document inside once window/tab (so we can refresh only one frame rather than whole root document and/or embed another page inside root document.

However, the <iframe> element is still available, and is more flexible, so it can be used to reproduce the <frameset> <frame> behaviour: <iframe> embed also html page inside root document, but is handle as another element container... it's a kind of float <frame>, not requiring a <frameset> parent and not requiring a specific frameset <!DOCTYPE> (but one for an html version supporting <iframe> onviously ^^)

Answer 2

Framset is not used anymore, so no need to worry about it.