If you are in business or going into business, you'll probably want a website.
However not everyone will need one.
Some businesses like Plumbers and other similar trades may benefit from having their details listed on existing websites, directories for example, but there's little need for smaller concerns to invest in a site of their own.
If you fit into this catagory, there's lots free, and paid, directory sites where you can get your business listed.
There are some trades that may benefit from a website, for example, a website would be a great place for a garden landscaping company to show its past work.
As a rule of thumb, if people in your industry have websites, then you probably should too, but you need not invest alot to get a web presence of a good standard.
If your competitors have websites, then it may pay you to make a modest investment in your own so that people know you are there.
If you have information that you cannot easily fit onto you business cards, business stationary, or vehicle livery, then a website is a great place to hold this information, then you simply mention your website where needed.
Obviously, in some cases, people will look solely online for certain services, so it can be critical to have an online presence.
Depending on what kind of business you run, you may find it helpfull to place help & support information relating to your products or services. This will help to reduce the need for clients to contact you directly, and to enabled a resource for support out of hours.
If you want to actually sell online then obviously a website is the answer.
If you just want to have a simple online presence you could start with a static website, that is, one that doesn't involve interaction with users or a database.
These are sometimes refered to as brosure sites.
Some interactions with users and/or databases include: forms that are filled in and processed, online stores, forums, etc
Selling online is an option you really should consider if you are a retailer, as an increasing amount of money is spent online.
If you do want to sell online you have a number of options regarging taking payment. Some payment processers require a special merchant account, others like PayPal can be utilised by smaller concerns without a merchant account.