Because test is "active" from the hour you inject it. It starts releasing into your bloodstream right way, but only as a trickle (the blob of oil known as a "depot" is gradually reduced in size as it is absorbed into the bloodstream and the ester cleaved off). So you don't get the lethargy associated with oral-only cycles. And once you stop taking the oral after 4 weeks or whatever, the real gains are just starting (so you won't have put the strain of an oral on your body for pretty much no reason).