Security
The core code of Drupal is heavily tested by the Drupal core team, Drupal community & the general public & the feedback then goes back so it can be improved - this cycle is never ending. A private CMS or a bespoke CMS doesn't have anywhere near the numbers that input their knowledge & skills in to the security of the CMS or the number of people that can check & give feedback compared to the open source CMS Drupal.

There are several CMS (Content Management Systems) available including Joomla, WordPress & Drupal. Drupal is a nice solid CMS & simultaneously it's simple to manage. Here are a few more aspects as to why Drupal is so nice in no particular order:

This is of a couple of reasons why Barack Obama & the Whitehouse sites are now partly using Drupal.
here is a dedicated security team and a process. When someone discovers a security flaw, instead of posting it in the module’s issue queue, as per normal, they notify the security team directly. The flaw itself is only made public when there’s a fix – for security reasons. Security announcements can be seen via email, RSS feed, or the website.

SEO
SEO is a large topic, but I will touch on two aspects here that Drupal can do with ease; Search engines like Google take into account the website page url and with Drupal it is easy to apply friendly urls that consist of having the page title as wording rather than a page id number. If you have the page or blog post matching the key-phrase that online users type in q Google search – then it is considered to be a benefit to your website in regards to the placement on the search engine rankings.


Page Optimization
Drupal has default caching features using the database to help to load the page quickly. Drupal can compress the CSS and Javascript which will help to load the page quickly for the users.

Benefits of Open Source
I’ve already mentioned that security is one of the benefits of open source. The community is a hub of pooled information that are willing to share knowledge and ideas as well as help each other out, particularly to people that are new to Drupal.
A key strength of Drupal is it’s a framework that can be powerfully extended, and that’s where user-contributed modules come in. A number of current core features started life as contributed modules. Drupal developers have found it invaluable to be able to use this resource available to their disposal and contributed to the community with modules they have developed themselves.

Drupal Framework
Drupal’s framework is well structured, adaptable, modular, expandable and is flexible in many different ways. The core framework allows modules and themes to be implemented easily with no required changes to the core framework or editing existing code. It’s setup so well, that any Drupal developer could take your current Drupal site and make new changes to the design or code even if they have not worked on the existing website before. This is because it has the universal framework and with it’s flexibility it is easy to make changes. This is not always the case with a private or bespoke CMS. Drupal has multi-tiered user permissions allowing for a group to work on the site with each user having the appropriate permissions for their role.

Drupal’s Content Construction Kit (CCK)
Drupal comes with content types like web page and blog. For each content type, you can easily ‘create content’ and submit a new web page, blog entry, etc. If you want a different content type or it doesn’t match the requirements you need, for example if you would like to make a collection of quotations, and you want to track the author’s name and the source of that quotation (e.g. book title) it is possible. This is where CCK comes into play with Drupal together as CCK can add new fields to the content types to meet your requirements.
CCK is currently a contributed module for Drupal version 6, but as it’s so good it will be a core module in the Drupal version 7.

Views
Views and CCK can work well separately, but together they work as a powerful combination. For example, CCK creates a content-type you want, for example collection of quotations, then Views can do a query builder for displaying and filtering results, lets say to display the latest 10 quotations that are about Drupal.