Do I Need a Domain Name?
That depends upon your purposes.
If you just want to put up a personal web page, you're probably better off simply placing that page with your current service provider. Practically all Internet dial-up accounts include a free web page. Your service provider can help you with the details.
If you're in business and wish to advertise your services on the Net, or provide information for clients, etc., it may still be simplest and most economical to set up the page(s) on an ordinary dial-up account. Unless your website is very large or will attract large numbers of users, this is usually OK, though you should check your service provider's terms of service.
The disadvantage of this inexpensive approach is that your site's address will carry your service provider's domain name, and probably won't be particularly memorable to your customers. If your account were email@example.com, your web page address would look something like: http://www.serviceprovider.com/~jjones. For most people, that address is not easily remembered.
It is a marketing maxim that if you're advertising to the general public, it is vitally important to present a simple, memorable message. Web addresses are no exception to this. Thus a well-chosen domain name can be absolutely essential to your enterprise on the Internet.
How Do I Decide if It's Worthwhile?
For most, it's a simple enough decision.
If you're going into business on the Net in a serious way, and especially if you're advertising to the public, you need your own domain name.
If you intend to build a huge website, or one that is intended to attract hundreds or thousands of users every day, you will need your own domain name, and have it hosted by a provider whose facilities are up to the task.
If for any reason you need some particular, recognizable name as your site's address, the only means to do so is to own your own domain name.
Sometimes someone simply wants a vanity address, for e-mail and/or a personal web page. If you're totally determined to be firstname.lastname@example.org (and no one else has registered the domain name), you can certainly do so, though at a much higher cost than an ordinary dial-up account.
It's all a matter of whether the expense (see below) is worth it to you.
What Does It Cost?
There are several costs involved in setting up a working domain.
We presently charges $10 for the first two years' registration of a new name, upon initial registration.
A good place to look for an IPP is THE LIST at http://www.thelist.com. It's a good idea to spend some time examining your options.