These columns are designed to replace the sender and recipient columns for people who mix their incoming and outgoing mails in one folder.
See http://www.ggbs.de/extensions/ShowInOut.html for a description of how to change the appearance of incoming and outgoing mails, e.g. show outgoing mails in italics (like Eudora, but see the FAQ!)
Most of the functionallity of this add-on is now standard since Thunderbird 45
For bugfixes see the homepage (http://www.ggbs.de/extensions/)