We're a small team of only two people, but we're tackling more and more rather big TYPO3 projects. Over time we introduced package management for PHP (composer) and JS (bower) and depended on frontend building with Grunt. With rising complexity and importance of our projects we felt the demand for an efficient and safe way to deploy our websites on different servers.
We helped our selves with small self written deploy scripts that performed building on the target server, but you can't run node (for Grunt) or sass in every environment, and if building goes wrong the website was likely to be broken on the live server - so we needed a Build Server.
But solutions we looked at turned out to be quite time consuming (setting up and managing self hosted Gitlab and Gitlab-CI or a Jenkins server and runners) or costly.