There are many reasons to register multiple permutations of words and similar domain names. The important thing is to choose one domain for your web presence, and focus the others onto it.

The .com, .net and .org Trifecta

Strong buy. Domain names are scarce — there is only one of each of them in the world. If you get one and not another, you can almost guarantee someone will have snapped it up by the next time you look, and likely be waiting for you to make them an offer. Obtain the .com, .net and .org variants of your domain name if you can. Get the local country domains for whichever countries you operate in as well if you operate outside the USA, or represent a multinational. Get the specialized TLD relevant to your business, if applicable. Ignore .biz, .info and .name.

Multiple-Word Domains

In a multiple-word domain name, there is often a debate whether or not to separate the words with a hyphen. The solution: Get both if you can afford to, e.g. johnsmith.com and john-smith.com. Again, if your budget permits, obtain likely misspellings like jonsmith.com. It is important to remember that each is a separate piece of real estate that all computers on the Internet consider to be distinct, even though people may conflate them.

examplestore.com, exampleproducts.com, etc.

If your domain is example.com, it may seem like a worthwhile investment to also register variants like exampleproducts.com and examplestore.com, to boost your search engine rankings and offer customers alternate ways of finding your site. Do not bother with this.

I invite any SEO/SEM professional to produce hard numbers on the concrete benefits of doing this, versus the real costs of registering all the aforementioned variants and then managing them all, as well as the opportunity cost in lost business intelligence due to the fact that cookies can't traverse domains.

Furthermore, the DNS is hierarchical, which means you can stack labels on top of one another, from right to left, at no extra cost — like store.example.com or products.example.com.