I don't think it helps the low placement of placenta but it helps alot in relieving back pain or pelvic pain. I wore it through out the whole pregnancy and felt alot better wearing it (less pain). I am wearing it recently because my belly is too large and it does not fit anymore.
I think it depends on you whether you have any back or pelvic pain or whether you feel the belly is too heavy. But ppl said you should start wearing it from the 4th month and I did.