After high dose chemotherapy, is your marrow destroyed, so you would need a bone marrow transplant?

Not really like that: If the person needs high dose chemo to treat underlying acute leukemia or other kind of blood cancer for instance- the bone marrow transplant is aimed to cure the leukemia/blood cancer-not really to treat the destroyed marrow from chemo. Certain chemo however can cause myelodysplastic syndrome which later can transform into leukemia, and transplant-if possible- would be the treatment option.

Autologous BMT/SCT: Autologous bmt or stem cell transplant uses the patient's own bone marrow or stem cells to "repopulate" the bone marrow after giving high-doses of chemotherapy or radiation (hdc/rt). The actual treatment is provided by the hdc/rt to eradicate residual cancer cells and the "transplanted" bone marrow/stem cells are just to support the patient from the sure consequence of hdc/rt - marrow failure.