This question was asked in phone interview for Unix position ? I had quick formal reply "because of security purpose" but he wanted in depth answer, what it could be ?
There are a couple reasons why you hide files in unix, or any OS for that matter; but when you give a canned, open ended answer like "security purpose" you leave yourself open to the "could you please elaborate?" response. What security purpose?
If, on the other hand, you know a reason why you'd hide a file in unix, say "not clutter my terminal window when I do an ls command", or "that way I won't delete a critical config file when removing files with * ", then the interviewer can at least assume you know what you are talking about.
Sure some interviewers are looking for a particular answer, and if someone asks you a question that can have multiple answers depending on the situation, then I think is ok to say "it depends on the situation, what are you looking for?"
They would not be in plain site, thus
(a) not visible to ls or rm commands - and these are the typical tools used by users
(b) configuration and settings files
In general:
for safety sake to keep from deleting or modifying by mistake
to secure and group together
Think of it this way, you push open the door and walk into a room. You can now tell everything about the room, right? Did you step out of the way and see what is BEHIND the door? Perhaps something interesting or important! A hidden file can be the same thing - there to be found, if you remember to look for it.