Why Exchanging RST early tells each operator how well they are actually being heard, which is the practical basis for everything that follows in the QSO. If the report is weak or noisy, you can raise power, switch antennas, slow your CW or use phonetics and repeats; if signals are strong, you can drop power and chat normally. It is an efficiency habit, not a regulation, and it lets both stations tailor the contact to current propagation and noise conditions.
Watch out Award programs such as DXCC or WAS do not require a signal report to be exchanged, so the answer about making the contact count is wrong; and a subjective readability and strength report says nothing about frequency accuracy, so it cannot calibrate a display.
Signal report first = 'how do I need to talk to you?' It sets power, mode and repeats, not awards or calibration.