By linking to a specific url in with a parameter, an attacker could send you to his own website. On its own this is not so bad: the attacker could more easily link directly to his own website instead. But in combination with another attack, you could be sent to the Plone login form and login, then get redirected to the specific url, and then get a second redirect to the attacker website.

References:
https://nvd.nist.gov/vuln/detail/CVE-2017-1000484
https://plone.org/security/hotfix/20171128/an-open-redirection-when-calling-a-specific-url