My oncologist and I were interviewed about liquid biopsies that are used to find DNA markers that can help find a more specific treatment.
I've mentioned this before, but I think this explains it clearly. I've am currently having these blood tests every three weeks, but so far all the markers I've had mutated and disappeared. Hopefully they will identify a new mutation that they can treat.
https://www.mskcc.org/blog/some-people-cancer-simple-blood-test-can-be-lifesaver