The Mac OS X operating system is made up of many components. Things like CUPS for printing, Bonjour for local network Device Discovery, TCP/IP for network communication, USB Mass Storage Driver for flash drives and external driver, USB Human Interface Driver for a verity of mice and keyboards. There are hundreds if thousands of components.
One of those built-in Components is Apple Remote Desktop Client. Every Mac with 10.5, 10.6, 10.7 has Apple Remote Dekstop Client Built-In. It's just part of the operating system; you can find it in system prefrences sharing. It'd tighy next to printer sharing, remote access, internet sharing, file sharing, extra.
When enabled in system prfrences, sharing, Apple Remote Desktop Client, would typicly be used in a multi computer environment. Perhap one person would supports many uers remotely. Perhaps this user would proive technical support, answering questions, and provide training. OR perhaps one user has many computers; and they don't want to have to walk across the house or office to grab one file on the other computer.
While you may not use Apple Remote Desktop Client; it's probable a good idea to install the update. Apple Software Update may not offer you other updates important update untill you install all the udpates it currently offers you.
Even if it's a component that you currently do not use. Those updates can patch bugs, improve security, and improve performance of your computer as a whole.
Perhaps it has something to do with the NSA. The FBI can serve Apple with a national security letter to force them to modify their software to allow the NSA to hack into your computer. Releasing updates to the Apple Remote Desktop Client with a backdoor would be the easiest way for Apple to comply: the basic functionality that the NSA is already there. The NSA just needs the ability to access it worldwide from Fort Mead on any computer using OS X.
There is nothing to worry about though. Even though the NSA might be able to see everything on your Mac and watch everything that you do on the computer (or in the same room, via the iSight camera), this would just be a latent ability, only to be activated when the NSA determines that you (or anyone three degrees away from you in any way) is involved in terrorism, espionage, or nuclear proliferation ... or gun running, drug smuggling ... or tax evasion, or use of online adult websites, or a few other criminal activities as approved by the FISA court, with terminology defined by the NSA. Honest, law abiliding citizens need not worry. If you don't authorize the Apple Remote Desktop Client update, the terrorists will have won. Namaste.