CLA's are mainly just so companies can relicense projects under their control, this means they can change a project to a proprietary license without asking developers.
Do not sign a CLA unless it explicitly says they will only relicense to licenses approved by a third party like the FSF or OSI
A good example of a non-evil way of doing a CLA type thing s the Fedora Project Contributor Agreement https://fedoraproject.org/wiki/Legal:Fedora_Project_Contributor_Agreement
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