Further, if the issuer has an unexpected need for cash (bridge financing) they can quickly issue securities that have already been registered with the SEC, without going through the registration process again. This also helps a firm to avoid borrowing from commercial banks or other such entities, which is generally a more expensive proposition.
For example, assume a company has 500,000 shares of common stock that they wish to issue to the public at some time in the next 2 years. However, at the moment they only wish to issue 200,000 shares, holding the remaining 300,000 shares in reserve.
The company would be wise to shelf register/file all 500,000 shares now, rather than registering 200,000 shares now and having to go through the entire process again within a year or two. To be eligible for a shelf registration, all 500,000 shares must be of the same type or class, carry the same risk, provide shareholders with the same rights, etc.
This process of shelf filing/registration is allowed via SEC rule 415 and generally requires securities to be investment grade securities or be similar to securities that have already been issued by the same company, who has filed with the SEC in the past.