Limiting access to your blog(s)

i guess blog-reading is not opened to just everyone anymore. as i was trying to go blog-hopping-slash-reading today i came across a blog which cannot be read unless i am invited. that is strange and creating mysteries, building intense and curiosity. is there some sort of confidential matters in that blog. or is the blog becoming too personal i.e. like reading someone's diary. a diary is usually kept to oneself with no intention of letting others know the writings inside until one day when the owner of the diary forgets, keep the diary lying around, or when someone goes through your things looking for something and then found the diary and then read it. and so now i know that blogger has a feature to limit access to your blogs just like facebook or friendster, etc.