why would a bone marrow biopsy be needed for an unusally high platelet count

i'm no expert, but platelets are produced in bone marrow (http://www.ouhsc.edu/platelets/Platelets/platelets%20intro.html) and there are a number of conditions, some temporary, some lasting, that can cause the bone marrow to produce too many platelets (http://www.nhlbi.nih.gov/health//dci/Diseases/thrm/thrm_causes.html). Your doctor is probably trying to rule these things out and search for the root cause of the problem. Hang in there an lets see what comes back from the test. Its better to know if there's a problem and what it is, so you can start to work on fixing it!