Drupal provides a secure foundation for any site that requires complex functions and relations, receives high peak or sustained traffic, or contains a large variety or amount of content.
Drupal has become a very secure platform due to it having a long-lasting, abundantly used, open source code base. Since everything "under the hood" is able to be heavily scrutinized by its vast number of contributors and any possible security threats, any vulnerabilities are quickly found and secured. Security and performance improvements are regularly made to the Drupal code base and are easy to pull into existing sites with minimal complications.
Drupal is a content management system (CMS), which allows for the creation and management of simple content right out of the box. This content management occurs through the provided graphical user interface, which allows individuals without any website management training to be able to create and manage content with little effort. With little styling you could have a functional website in very little time.
What Drupal offers that many other CMSs do not is the ability to alter how content is managed and displayed based on any number of complex rules required for your specific site. Various contributed modules allow for easy adding of features that do not come in Drupal core such as additional field types or display formats and if you need something even more complex, developers can create their own modules specific to the site's needs.
High Traffic & Large Content:
One of the largest complaints about a website is that it is "too slow." This can be caused by several things ranging from client network issues to old and slow hosting servers. Drupal helps reduce this issue by capitalizing on the ability to cache as much of the site as it can.
Any time you are dealing with content that gets compiled, the server has to build the requested page and then send it back to the client. This is usually one of the largest bottlenecks for the overall site performance. With Drupal a cache version of the page is store when first accessed so that future occurrences can use the cached version instead of having to recompile the page. This reduces the sites over all server usage and allows for faster handling of requests allowing for more frequent and larger requests.