The main objective of creating a hoover is to decrease the boiling point of any moisture present in the system, causing it to evaporate. Additionally, it ensures the absence of air in the system, leaving only refrigerant occupying that area. Furthermore, when linked to the automated machinery used in automotive workshops, it serves the purpose of identifying any potential leaks in the system. However, it should be noted that this method is not the most effective, since a vacuum just represents the absence of atmospheric pressure (minus 1 bar). It is recommended to do a nitrogen pressure test to verify the integrity of the system. This test should be conducted at a minimum pressure equal to the system's operating pressure, namely on the high pressure side, which should be at least 15 bar. I would also be sceptical about using the do-it-yourself top up cans, since the refrigerant must be precisely measured and added once a hoover has been created, in order to get best performance. Using one of those cans involves mere speculation.