Can you explain to me in plain English why I should be concerned about security and privacy issues on my computer?

The same reasons you are concerned over unwanted people entering your property, peeping through your windows, or installing cameras in your house. These same physical rights apply directly to digital rights. You should not want unwanted people having access to your digital files, your photos, emails, website history, or your camera or microphone.  Your home is your private life, your digital life should have the same rights and protections.