If your computer seems to be working fine, you may wonder why you should apply a patch. By not applying a patch you might be leaving the door open for a malware attack. Malware exploits flaws in a system in order to do its work. In addition, the timeframe between an exploit and when a patch is released is continually getting shorter.
Defects in clients like web browsers, email programs, image viewers, instant messaging software, and media players may allow malicious websites, etc. to infect or compromise your computer with no action on your part other than viewing or listening to the website, message, or media.
What to patch
Not all of the vulnerabilities that exist in products or technologies will affect you. However, any software you use is a potential source of vulnerabilities that could lead to a compromise of security or identity. The more commonly used a program is, the bigger target it represents and the more likely it is that a vulnerability will be exploited.
For the more obscure software you use, contact the vendor to receive updates, patches, or vulnerability alerts. Additionally, don't forget to patch your anti-virus software.