The best way to add modules and themes to Drupal 8 is using Composer, which is a very powerful and awesome tool to manage dependencies. But all of this comes with a cost: just to add a module you have to install Composer, GIT, learn how to write a composer.json and so on. No more! Just choose the modules and themes you want and we do the rest.
Do you need a module or theme in your project and you remember just a part of the name? or you don't know what is the latest version? Of course you can use Google or drupal.org to find those information. With composy.io you can search by name and choose the version in a single interface.
Composer has to download a lot of files just to compute the dependency tree and then it has to download the actual Drupal code. If you are on a slow connection or behind a proxy the process could be very frustrating. We cache all the metadata and Drupal code and provide to you a single zip file to download. With composy.io you can even compose your website with your mobile, your build will be ready for you when you'll arrive office