Anytime you deal with confidential or private information (Social security numbers, unlisted telephone numbers, health information, etc) you need to take steps to keep such information confidential. Some types of information, if released without the owner's consent carry legal penalties. If confidential information is also released without consent and then used for improper purposes like identity theft, the information owner may seek compensation from the organization that gave out the information.
The disclosures you see at the bottoms of emails to you are attempts to reduce the likelihood of improper use of information. They also provide the organization that sent it out with an argument that it did everything they could to stop any improper use.
Although no lawyer can guarantee that type of disclaimer will have any actual legal defense value, it does make the clients/information owners feel better when they see it on emails. I recommend adding an email disclaimer to your email signature line.
If your business does take confidential information like credit card numbers, SSNs, or anything else that you think should be private and save the information in an electronic form or if you transfer paper forms by electronic means, you should consider having a company information and retention policy drafted. Well written policies give lawyers arguments if anything ever did happen and former clients were looking at your company for accidentally releasing certain types of information.