When you market your business and want someone to take a specific action (sign up for your newsletter, register for an event, purchase a service), it can be helpful to guide people to a page that is focused on that action. You can create a hidden page that's not visible to the public, but can be accessed through email marketing or online ads like Google or Facebook.
Sure you could drive people to regular pages on your website for this information, but by using targeted landing pages you can better refine the messaging to your target audience, track the metrics of your marketing efforts, test out responses to different images and words, and more.
Another reason to have these pages hidden is to offer exclusive content or promotions that you don’t want just anyone to stumble on who views your website. For example, You could have a special page with a coupon for newsletter subscribers, a page with an offer for wedding couples on their first anniversary, or even a special service that you only want to offer friends and family.