Implementing a Grid layout system would allow creating interfaces that is not currently supported through Flexbox.
In short this is because Flexbox is a one-dimensional system, while Grid is two-dimensional. This grants control over columns, which is especially useful in responsive web design. It is for example not currently possible to create an iterating container with a predefined number of columns and vertical direction (reading down). There are also several challenges linked with spacing in the current flexbox, that a grid-system would solve.
Some context:
CSS Grid vs. Flexbox: Which Should You Use and When? (tutsplus.com)
Difference between CSS Grid and CSS Flexbox - GeeksforGeeks
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