Juli Clover, MacRumors:

There are some major improvements to Siri in iOS 15, with Apple introducing features that iPhone users have long asked for. On devices with an A12 chip or later, ‌Siri‌ can do on-device processing and there’s support for offline requests.

Most importantly:

Starting in ‌iOS 15‌, speech processing and personalization are done on-device. This makes ‌Siri‌ faster at processing requests, but also more secure. Most audio requests made of ‌Siri‌ are kept entirely on the ‌iPhone‌ and are no longer uploaded to Apple’s servers for processing.

And:

With on-device processing now available, there are a wide range of ‌Siri‌ requests that can be handled offline. ‌Siri‌ can create (and disable) timers and alarms, launch apps, control audio playback, and access Settings options.

This is a huge step up, especially useful if you are in an area with low cell coverage. In a similar vein, wonder if Apple Maps will add the ability to download maps, something Google Maps offers in its Android and iPhone apps.