Here is the quotation from MacRumors:
The updated build may have been designed to address an issue some users have been experiencing with SIM card-related errors. With the update affecting a relatively small portion of users, Apple may have elected not to push it out to all users in the form of an iOS 5.0.2 software update.
Yes it's understandable move of Apple, the release the tweaked iOS firmware is certainly to respond the current and underway situation of iPhone 4S NO SIM Card Installed and No Services issues, and the A5-phone users have been complaining over the condition in the Apple support forum. And if you having the said problem too, you might be considering to update to this tweaked release.
Download iOS 5.0.1 (9A406) firmware for iPhone 4S
And if you need to download another iOS firmware version, you can browse our iOS firmware download archive page.