If you are trying to for example develop python and accessing something encrypted with SSl and that SSL certificate is not signed by a well known CA you might get an error.

That is because your CA is not added to the local CA store of e.g. Ubuntu.

You can add your signatures by:

sudo mkdir /usr/share/ca-certificates/extra
sudo cp FOO.crt /usr/share/ca-certificates/extra/FOO.crt
sudo dpkg-reconfigure ca-certificates

Then the new certificates will be added to your local store.
(please be careful as sudo dpkg-reconfigure ca-certificates is only checking for files *.crt, so no *.cer etc).