Joomla web sites allow a person to keep track of multiple content submissions easily. Most people use standard templates which do not cause their site to look bad, but they also fail to distinguish the site in anyway. Someone who understands the basics of HTML and other scripting languages may decided to make alterations to the site by himself.
Self-made alterations are acceptable for many circumstances, but a person who wants to put forth a truly professional website may not want to show off his own work, unless he knows what he is doing. The popularity of Joomla and its wide spread usage gives a website owner another option. Not everyone has the art for graphic design or the time and patience for website design. Web site owners often focus more on the business end of their site. When a person wants to put forth a good image but cannot design his site as he would like to, he should hire someone to design a professional template.
The professional template designers will cost more than the ready-made templates many people use, but a custom template design will match the needs of a business exactly. Getting a ready-made template allows the user to make adjustments, but it may not be exactly what a user needs in the long run. Rather than getting something that meets a person's needs almost exactly, a person can buy a customized template that has the features he needs without having to settle for something that is second best.
The above statements could apply to almost any item that is custom made and not just Joomla. After all, don't custom-tailored clothes fit better than clothes bought off the rack? The principle applies to computer software, just as much as it does to anything else in life.