In Windows 7, Microsoft significantly enhanced the accessibility support with many features along with the addition of Windows Automation API 3.0. This new API enhancement include performance improvements and bug fixes too. To ensure users of earlier version of Windows get this benefit too, Microsoft released these improvements as down-level optional Platform Update for Windows XP and onwards.
The Coded UI Test and Action Recording of MTM are heavily dependent on accessibility. So if you are using these features, it is advisable that you install this Platform Update. And this update is absolutely must if you are testing WPF application using Coded UI Test or MTM otherwise you may experience hangs in some scenarios.
More information on this Platform Update can be found here.