“Software that implements TLS typically (altough not always) will write keys and other TLS secrets to this file.”
Set one environment variable, SSLKEYLOGFILE, and Chrome, Firefox and curl will dump their session keys to a text file. Point Wireshark at it and every HTTPS conversation on your machine is readable. That is a gift for anyone reverse engineering what their apps send home. It is also a reminder that anything with access to your environment can read your encrypted traffic.