It is for two reasons. By far, the primary concern is that UV rays will make the textiles deteriorate over time. UV resistant textiles will last a lot longer, and deteriorate less from only sunlight.
It is also a minor concern for the people inside, but only if they will be spending considerable amounts of time in the tent in direct sunlight. Similar to how its possible to get a sunburn through thin clothing if you are out long enough, a small amount of UV rays will pass through a non-UV resistant tent and reach the people inside. Usually this isn't a concern, but if you spend all day every day in the tent without sunscreen, then you would have the potential to get tanned or burned