As a radiographer I can say it is so, so useful to have a bit of what you're looking for to x-ray at the same time. You can usually see glass but I remember one case as a student where someone came in with a 5-6 inch shard of glass from a shed window protruding from her eye. Wanted an x-ray to see how far it went in before they removed it. The x-ray showed nothing. Same goes for any other foreign objects, especially inhaled/ingested ones as these can be real buggers to see. You can also x-ray them first to demonstrate to cas officers who are insisting you x-ray that they really won't see that missing lego brick that Jimmy may or may not (usually the latter) have swallowed.