CT scans just show organ shapes and densities. They don't light up a special colour or anything like that for mets (unlike PSMA PET/CT scans, which do). Mets are therefore identified in CT scans by misshapen organs, or unexpected density changes in organs. Some organs don't show up well in CT scans, and if they're particularly looking at those, then a contrast can help to show up things like blood vessels, which often grow differently in tumours.
So I think the answer is, it depends why they're doing the CT scan. Bones show up really well on CT scans and probably don't need contrast, whereas some soft organs would. However, I don't know this area very well.