Firmware updates can be used to improve the performance of products, fix problems or add extra functionality. Those products connected to the internet will usually do this update automatically, while other vision products – set-top boxes and the like – can be sent 'over the air' updates piggybacked on the broadcast signal.
How worthwhile is it doing these updates? Well, it depends what the update brings: for example,
the latest one for your series of TVs merely improves the set's performance with conditional access modules - the kind of smart card reader used in some regions for pay-TV. So probably not worth doing unless you use such a system.
However, if you haven't ever updated since buying the set, could be you've missed out on some more significant updates: it's worth using the
update-checking routine described on the Sony website to see how far adrift you are of the latest firmware.
Is the update process likely to give your set a case of TVflu? Unlikely, assuming your computer and drive are scanned for viruses regularly, and even then only a virus targeted directly at the TV is going to cause any problem, and those are very rare. Virus-writers can cause mushc more damage elsewhere, so it's not worth their effort to target a few TVs.