Joomla is one of the free CMS (Content Management Systems) that became very populars and created a comunity around it during its several years of existance. The number of websites created with Joomla is numbered in a few millions, so one can say it's one of the most popular tools for that. The next reasons are only a few why you shoold use Joomla for your website design. You only need a web server that handles php and a mysql database to install this CMS and, after configuring it, to have a functional website. Next you have to add some functions (using the components, modules and plugins) you need and select or create a Joomla theme.
It is user friendly and easy to learn
Even beginners are welcome to use this - it may take longer to find out about all its function but it's easy to learn. It takes at most a few days to learn it all over. The CMS is rather big (especially when you never worked with other CMSs) because it's under constant development for some years, but it's easy to follow the modification made in time - there are availabe online various versions of Joomla.
It's a free Content Managment System
You can download for free, so this would be in your advantage. You wouldn't be wasting money for an application that you wouldn't even know how to use. The recommendation it to spend money on its customization (especially as a beginner, or when you want your site optimized to its best) and manteinance - but it's a lot less money than creating a website from scratch. Use experts in Joomla, or at least someone that have already worked with it for a few months or years.
It created a community that offers support
The constant use and improvement of Joomla triggered an increase number of users with expertise in different degres about this CMS, its structure and extensions. If you have problems (using, configuring or extending Joomla) you can search for responses with any search engines - there are great chances to find them. Even if you don't find any good respons for your problem, you can always ask. Most of the online support is free, but in some cases you can pay for it (like an online subscription to some websites or for experts' responses).
It's extensible and flexible - it gives users control
Joomla was built to be enhanced. When you download and install the packege from Joomla development website you only get the main - basic package. There are thounsands of components, modules and plugins you can add this basic package from Joomla Extension Directory - some of them for free, some for a price. You have to know exactly what you want from your future website before adding any new script to your Content Management System.
It's secure by default
Joomla is built to be secure in most environments, but you must understand that Internet security is a vast and fast-moving topic. No set of rules can cover all possible situations, especially if new types of attack are being developed every year. This means you have to extend Joomla security with server security (I mean your server security).
Because Joomla is being upgraided constantly and the developers add new functions or fix bugs (even security bugs) you should always update you Joomla version and the extensions (components, modules and plugins) you have installed on your website.