These Realtek HD audio drivers are provided free by Realtek. They are intended to allow Windows to communicate with Realtek audio hardware, and so there they are only useful to people who already own said hardware. To that end, there is no point in charging a fee for these drivers.
A Realtek HD audio driver is software that enables your operating system (in this case, Windows) to communicate with your Realtek audio hardware. Drivers are essential for making full use of your computer hardware, and not having proper audio drivers installed on your system would mean no sound at all.
As long as they are downloaded from a trusted and reputable source, the drivers are perfectly safe. You should always exercise caution when downloading files from the Internet. If in doubt, Realtek's website has a download section with the latest version of the driver, as well as all previous versions.
These particular drivers are indeed the 64 bit variants. This means they are designed to work on 64 bit operating systems specifically, and will not function correctly—if at all—on 32 bit systems. This goes likewise for 64 bit drivers on a 32 bit system. 64 bit systems typically have stricter security than 32 bit systems.
These HD audio drivers are designed to work with Windows Vista, however they will typically be compatible with Windows 7 due to the similar nature of the operating systems. They almost certainly won't work on older versions of Windows. Newer versions of the drivers are available for more recent operating systems.
The chances are, your Windows installation will have installed some functional drivers that work fine. In that respect, you don't need these drivers. Sometimes, however, there are performance and compatibility issues that may necessitate downloading the Realtek drivers. Ultimately it comes down to whether your audio is working or not.
The simplest way to update your audio drivers is allow Windows to automatically update them. If this is not working, however, you can try updating the drivers manually. If Windows cannot find a newer version of the drivers itself, you will need to download them and specify where they are.
When downloading drivers, or any files from the Internet for that matter, always be cautious. Malicious software can easily find its way onto your system if you're not careful. Reputable file repository sites are usually fine, but check comments and download numbers. The official Realtek website is also an option.
Many drivers come in an executable format, meaning you can simply download them, run them, and leave the installer to do the rest. For drivers that do not have an installer, you will need to install the driver through Windows Device Manager, specifying the exact location of the driver files.
With a modern Windows machine, there is a good chance that you already have fully functional audio drivers installed. In some cases, driver variants may be installed by the manufacturer that contain features specific to that machine. The short answer is; if your audio is working, you don't need these.