Waiting for colonoscopy . Why do mri and ct scan first?

I have had reoccurring blood in stools, watery poo and pain for 18month. I have repeatedly spoken to my GP who hasn't taken it seriously. I had blood tests done twice over this time( both normal) and also stool samples (also normal) . I have had periods in between where I have been much better and felt almost normal, but then it starts again.

Finally got to see a doctor who did an exam and felt a mass. I was sent to a consultant who also said he felt the mass. I have had an MRI and CT scan an now waiting for a colonoscopy.  The waiting is torture.  I'm not even 40 and have a young child. I've no idea why the scans have been done firstly and my brain just thinks its going to be awful news.