What Is The Purpose Of A Trans Rectal Prostate Ultrasound?

I have a urology question. I went to see my doctor about a vibrating sensation in my penis and groin which I thought might be referral sensations coming from my nerve root irritation problems in my back and I wanted to rule out anything urological. The doctor checked my scrotum and said it felt normal and checked my prostate by a rectal digital exam and said the prostate felt normal. He gave me a requisition for a scrotum ultrasound which he said I could use if I still felt worried. At the last second, he also checked the box for a transrectal ultrasound (not for a biopsy) of the prostate because I told him that I had been urinating frequently that day. I did some research online and read that the transrectal prostate ultrasound is usually only done after an abnormal finding from a digital exam or a high PSA reading (PSA test was not done), or to guide the needle in a biopsy. It sounds like this type of prostate ultrasound is not a common screening method. I am worried about the transrectal ultrasound of my prostate is unnecessary in my case and the possibility of false positive results which I have heard are common with the prostate. What do you think would be the reason for my doctor ordering that transrectal prostate ultrasound? What are the general purposes, pros and cons, and risks of having a prostate ultrasound? I would appreciate your response as I am very conflicted about whether to have that prostate ultrasound done. Thanks.