How do you display grid on Artboard and why would you do that?

The grid can be displayed using Artboard propery inspector.

You can use either Layout grid or Square grid, just check the Show/Hide Grid checkbox and select the grid type in the dropdown list.

When the grid is shown, it is way easier to place screen elements to their precise locations and keep them well aligned. Grids enable designers to manage the proportions between UI elements, such as spacing and margins. This helps to create pixel-perfect designs from the start and avoid timely reworking caused by incorrect adjustments.