A license for MailChimp for WordPress gives you access to 1 year of plugin updates and priority support.
You can continue to use the plugin after your license expires, but you will no longer be eligible for email support and upgrades.
Plugin updates contain new features, bug fixes, improvements and ensured compatibility with the latest versions of both MailChimp and WordPress. We always recommend you stick with the latest version of the plugin.
If you currently do not have an active license subscription, read how to renew your license here.