They testified that they used the phones to hide their affair.
“The predominant reason that we communicated on our work phones was because we were trying to keep our affair a secret from our spouses," Page told the IG.
http://www.foxnews.com/politics/2018/06/14/fbis-anti-trump-lovebirds-page-and-strzok-used-work-phones-to-shield-affair-from-spouses-ig-report.html