A CSS Grid is a layout that saves time with responsive design and development. Having a strong understanding of core CSS will allow you to easily create responsive user interfaces without needing to use Bootstrap and allowing you to design with core CSS.
Why CSS Grid?
CSS Grid is going to be the standard when moving forward with website design. CSS Girds is a great way to present a website, however, it may run into issues with older browsers, however, that is starting to end.
Unlike Bootstrap which is a third party framework, CSS Grid is a core framework and is overall considered a better alternative then Bootstrap. Favoring core technologies is an advancement because you are always assured the technology will continue to advance whereas third part applications risk losing popularity