A Rolling Release Cycle
A rolling release cycle dispenses with regular, standard Linux distribution releases. For example, Arch Linux uses a rolling release cycle. There aren’t multiple different releases of Arch. Instead, there’s just a single version of Arch. Software packages are tested and then released immediately to the stable version of the Linux distribution. Depending on your distribution, they may not even see much testing before they’re released as stable updates. When a new version of an application or system utility is released, it will head straight to the current Linux distribution. A rolling release distribution is never “frozen in time” — instead, it’s updated on a rolling basis.
A Standard Release Cycle
Most Linux distributions use standard release cycles. For example, Ubuntu uses standard releases — these may also be called point releases or stable releases. The Ubuntu project regularly release new versions of Ubuntu every six months. During the six-month development process, they take the latest versions of all the software in their repositories and package it up, updating all the software. They then “freeze” the versions of the software in the Ubuntu repositories and spend a few months testing it, making sure all the software versions work well together and fixing bugs.
source: https://www.howtogeek.com/192939/linux-distribution-basics-rolling-releases-vs.-standard-releases/
edit: @Caliga and other users suggested a great idea! To have more channels and let the user choose for himself how often and how reliable he wants the updates he will receive to be, while keeping PureOS a Rolling Release distro. An example of 3 different channels: 'unstable’, ‘testing’ and ‘stable’.