One advantage of utilizing the Grid component instead of directly employing flexbox is the ease of adjusting grid columns for various screen sizes using theme-based breakpoint props (xs, sm, md, etc.). If CSS Grid is your preference (and you're not concerned about supporting IE 11) or if you prefer utilizing flexbox directly, there's no reason not to go ahead and use it. Material-UI doesn't take into account whether or not you're utilizing the Grid component.
In my own experience, there are numerous instances in which Material-UI's Grid
component effortlessly achieves the desired layout, so we opt to utilize it. However, there are also scenarios where Grid
might hinder us, leading us to resort to using flexbox CSS directly (currently, our application still supports IE 11, preventing us from using CSS Grid. I anticipate a future point where we can update our supported browsers and transition to CSS Grid).