Having perused the official changelog, it's fixes for an awful lot of background services and modules, most - though not all - of which are Win32-related. However, in today's malware-strewn and fraudulent times, we should never disregard security fixes though, so update today in all affected mobiles, just in case!
The Release Preview (RP) ring also gets this update, with the build number jumping up accordingly. We should still expect yet another new RP build in a few weeks though.
Note that the Release Preview ring on the Insiders programme has assumed greater significance than ever before with Microsoft's decision not to update many still active Windows Phones to the Creators Update (branch 1703) of Windows 10. This includes the Lumia 930 and 1520, though also many other phones (735, 830, etc.), including those hacked up to the Creators Update branch in the first place. So today's new build of the OS - 15063.540 - for the Release Preview ring is well worth grabbing for all these phones.
As ever, with all phones, head into Settings/Update & Security/Phone update in order to get the update immediately.