It's from the Guardian Open Platform[1], which has a clause in the TOS[2] requiring that that the "Powered By The Guardian" logo be displayed where you display Guardian content

Include a "Powered by The Guardian" logo (or such other Guardian logo as we may require from time to time) on the same webpage as any republished OP Content, or any tool or function that is based on OP Content. Such logo must be a reproduction of the "Powered By" file found at http://www.theguardian.com/open-platform/logos, and comply with any special terms set out by us. The "Powered by The Guardian" logo must not be used in conjunction with any content other than the OP Content.

[1] https://open-platform.theguardian.com/

[2] https://www.theguardian.com/open-platform/terms-and-conditio...