As problematic as they can be, there are cases (including two mentioned on Mayo Connect, I believe) where tumor markers were the only sign that something was going wrong. I recall one writer here noting that they saved her life as they led to deeper examination and PET (or MRI) scan that found distant recurrence.

I wish that there were more consistent methodology for monitoring this stuff as relying solely on mammo or ultrasound seems woefully inadequate for identifying distant cancers early.