Yes, in some cases grouting is necessary:
Facades - here grouting is required to prevent water and UV rays from getting under the slabs, which could weaken the adhesive bond.
Rooms exposed to moisture - such as bathrooms, kitchens, or areas with direct contact with water, should also be grouted to protect the slabs.
In other cases grouting is not mandatory, but some customers decide to do it to achieve a specific look or greater protection.