Composer is a dependency manager for PHP, the programming language is written in. It allows to automatically install and update third-party packages and libraries that are used in an application together with their dependencies. Composer can also be used to update Drupal core and contributed modules. Reusing third-party code from other developers reduces costs and provides access to mature solutions that have been tested and improved by many programmers.
Starting a new Drupal 8 project with the LakeDrops composer template only takes less than 3 minutes. And in that short period of time you're note only getting the code base but also a number of extra benefits:
As a team that had spend time and money for years in developing best practices for staging, deployment and operations of many Drupal sites on version 5 to 7, the all new Drupal 8 came with some surprises and challenges in 2015, when first alpha and then beta releases started to see the light. Where all of Drupal core up to Drupal 7 had been exposed in the main project folder, all the contributed code from the Drupal community and third parties was located deep down in the code tree with custom code for the specific project sometimes was hard to separate from other contributions.
Drupal 8 projects are in most cases controlled by Composer and following best practices, the directory structure of a Drupal project looks similar to this (more details have already been covered here):