A Disclaimer, sometimes referred to as a Disclosure, is a specific statement for visitors in order to provide transparency. For example, as a website owner, you disclose that your site blog posts contain affiliate links (since it's required by the US FTC and major affiliate networks).
You need a Disclaimer or a Disclosure if it makes sense. If it's required by law (FTC) or by an affiliate network (such as Commission Junction), then you need it. If not, you can include this type of statement on your website as a sign of good faith towards your users. For example, you can disclose to your site visitors that you have no responsibility over third party sites (called "External Links Disclaimer").
Generally, no. However, a Disclaimer is required by major affiliate networks such as Amazon Associates or Commission Junction if you have affiliate links on your website.