The W3C's HTML Techniques for Web Content Accessibility Guidelines 1.0 notes
fieldset as a tag you can use to semantically group fields in a form. You can then label that fieldset with a
legend. Depending on how the browser accessing your content works, this could increase accessibility (for instance, a screen reader could indicate that the following section of the form is for alternate shipping addresses, and that would let the visitor know they could possibly skip that section).
You can use a fieldset whenever you want, even if you only have one field. It depends on how you want to semantically describe your content and how concerned you are with making your forms accessible. For instance, a search form containing an input field and submit button could be contained within a fieldset and use a legend, but you could use CSS to hide both and display the form however you want. It's up to you how you want to denote that, including whether you want to use it for input/edit pages or display/details pages.