From WP Tavern, the decision to sponsor a full-time contributor:
SiteGround identified and selected its new contributor based on recommendations from others involved in core development. “In search of developing our WordPress involvement even further, we made some inquiries as to what is most needed by the project at the moment,” said Tina Kesova, VP of Strategic Partnerships at SiteGround.
This is great news. SiteGround has gotten really involved in the WordPress community, and I'm proud to see them supporting a full-time contributor in addition to their other community efforts. I think things like this pay off for the companies involved in the long run when done right.