A domain extension: A top-level domain (TLD) is the suffix that follows the domain name in a web address.
How many domain name extensions you want to buy depends on how big your business is, so how much protection do you want for your website? For example, a small business may just want to purchase a .com or a country extension (In New Zealand the country example is .nz or .co.nz)
A large company many want to protect themselves further for example if someone searches afirmo.com OR afirmo.co.nz OR afirmo.nz they can be redirected to the main website OR simply not connect but they are all owned by Afirmo. This means no-one else could set up a domain/website with the same company/business name.
In the this example, I have searched for mybusinessname and the results show that the .com has already been purchased but the other extensions are available to be purchased
NB If you do buy other domain extensions and variations, you can redirect them to your main website.