Use this in written business correspondence (email, letters, reports) when you want to state a polite presumption about the reader's situation or about a shared understanding — essentially a humble, formal version of ～と思います. It very often appears in the opening or closing of a message, typically as ～ことと存じますが／～ことと存じますので, softening a request or a piece of news. Avoid it in speech except in highly ceremonial settings, and avoid it for your own actions (存じます is the humble form of 思う／知る, so the content is normally about the other party or a general state of affairs).