The Beta Software Program has been open for some time, but as 9to5Mac points out, Apple has begun to more directly ask users to check out its beta releases. The more people Apple gets to test out builds of its new operating systems, the easier it should be to locate and understand potential bugs and other hiccups.
Apple
This makes sense, since iOS 15 and iPadOS 15, watchOS 8, tvOS 15, and macOS Monterey are all planned to release this fall.