well, first of all, because @vhymanskyy said so! :))
but seriously, when using the standard method, every time the esp wakes up, has to run blynk.begin, has to connect to the server, wait for some stuff to happen, has to execute blynk.run, etc. all these steps can take quite some time
with the api approach, you eliminate the blynk.begin and blynk.run from the routine, and the esp opens a direct http request to the server, updates the values and goes back to sleep. i’m not an expert in http and all, but this can be executed somewhat faster than the standard method.
if you read ALL the the topic i have linked, there are links and better explanations.