TGM Plugin Activation is a PHP library that allows you to easily require or recommend plugins for your WordPress themes (and plugins). It allows your users to install, update and even automatically activate plugins in singular or bulk fashion using native WordPress classes, functions and interfaces. You can reference bundled plugins, plugins from the WordPress Plugin Repository or even plugins hosted elsewhere on the internet.
While the builds run for TGMPA are still very small, once unit tests start being added, this will make a difference. In the mean time, it doesn't hurt having this directive in the script.
As suggested by @johnbillion in https://johnblackbourn.com/reducing-travis-ci-build-times-for-wordpress-projects/ , when not creating code coverage reports, Xdebug is not needed and disabling it will speed up the build.
While the builds run for TGMPA are still very small, once unit tests start being added, this will make a difference. In the mean time, it doesn't hurt having this directive in the script.