Build 10586.682 was compiled on November 8th and is exclusively available to devices still running on Threshold 2.
Threshold was the codename for the first development cycle which ended in the first public release of Windows 10 Mobile with Threshold 2.
Most Windows 10 eligible devices already are running with the Anniversary Update aka Redstone 1. This means that Microsoft decided to deliver a Pre-Redstone update to a quite limited number of handsets.
Those are either Windows 10 uneligible models like the Lumia 920 and Lumia 1020 which got early access to Windows 10 Mobile via the Insider Program. (Microsoft later excluded those models for the final release), or handsets which are still waiting for the Anniversary Update.
Presumably, the update includes minor performance improvements and bug fixes, as it the case with most other cumulative updates. A new feature usually gets the proper announcement which is not the case here, making this option rather unlikely.
The Windows 10 Mobile stall
Perhaps the new build has something to do with the stalled progress of Windows 10 Mobile. According to recent data, the OS is simply not finding its way to new devices quickly enough. As of August, it is running on only 14% of all Windows phone devices.
Microsoft got a lot of criticism as it made the platform available to only to a limited number of older Windows devices in March this year.
The Windows 10 Anniversary Update, also known as Windows 10 Summer Update, rolled out on August 2nd. Windows 10 PC, HoloLens, and Xbox were immediately affected while Windows Phone owners had to wait two weeks longer to receive the update.