Template Manager: Subpage and Frontpage

Hello = )
What is the difference in these two?

Subpage is the default, frontpage is assigned.
They both look the same when I open them but frontpage is the one which when I make changes to, my site changes. So what is subpage for?

If you go to blog category you will see different menu items, this is why we need to duplicate templates. There are no differences between this two templates in template manager, only default menu item is different.