Every helicopter needs a port, and assuming your web site is a helicopter (work with me), you need somewhere for your pilots to land. If not, they will just hover in midair, their productivity completely shot, until they run out of gas and fall out of the sky.
What this perhaps too graphic example indicates is that every business needs a place to call home. The customers looking to patronize a business must have some sense of comfort about the business, some sense that it will be there if needed. Especially if your business is selling products that are expected to last for a long time and may require maintenance from the manufacturer, or if you give any kind of warranty with your product, people will simply expect a landing page. This is why long term businesses do not base their web presence on free WordPress or eBlogger pages.
The purpose of a landing page and when to use a landing page is also to have a place to drive customers from many different advertising vantages. If your marketing is done right, you will have customers come in from offline advertising, word of mouth, online advertising, and the public relations that others will do for you (becuase your product or service is just that good). They need to have some place to tell people to go; that would be your landing page.
You also want a landing page for the search engine algorithms. As you drive traffic to your landing page, it creates a sense of familiarity with a search engine spider, which will then give your page popularity and seniority points, both of which raise that page in the search engine rankings.
As well, if others link to your landing page, you gain backlink points, the most important criteria for search engine rankings. If you do not have a landing page for your business, these hits which would otherwise drive search engine placement up and expose you to even more customers will be interspersed between many pages, offering you little benefit in the long run.
In short, you do your customers and the search engines a huge favor by creating and maintaining a landing page from which to base your business operations. And who doesn’t like to receive huge favors? It is the professional business which tends to its customer base like this, and this is why businesses with landing pages simply last longer and do much more business than those without.
Recent innovations have made creating landing pages easy with products such as a wordpress specific landing age plugin.