Why are .thunderbird and .evolution hidden ?

[Marcel Rieux]

I don't have much important data and I do my back-ups in the most
simple way by saving my /home directory except for a few directories
to a DVD. But K3B asks if i want to save hidden files. If I answer no,
the .thunderbird and .evolution directories aren't saved and there's
some important data in there. If I answer yes, there then is a sleuth
of hidden files that are saved that I don't really need.

One way around this is to copy those 2 directories and then rename
them without the beginning ".". This way you miss a few files or
directories if you don't also rename them -- such as .#evolution.sbd
and .parentlock -- which... might not be important -- but why the hell
are .thunderbird and .evolution hidden in the first place?

Thanks!