The efficiency of a power supply is the percentage of the power that is pulled directly from a power outlet that is passed through to the components in the computer. For example: the components in a system use a total of 400 watt. The power supply has an efficiency of 80%, and as such it uses 400 / 0.80 = 500 watt of power. In comparison: the same system with a power supply that has an efficiency of 90% only uses 400 / 0.90 = 444 watt of power. This could potentially make a difference in the utility bill, but can also be seen elsewhere. After all this lost power (with the exception of the power used for the fan) translates itself to generated heat. The more efficient the power supply, the less heat it generates. In other words this means that the power supply needs less cooling and has a longer expected lifetime.
The efficiency of the power supplies is very good in all cases, but what stands out is that when we compare the results of the Seasonic Prime Gold and Platinum models, is that the Platinum models do not perform any better than the cheaper Gold-versions. And that while the only difference between both series is supposed to be the efficiency. After contact with Seasonic we now know that the Platinum-label should be seen as a guarantee that the power supplies at least meet the requirements of the 80 Plus Platinum, while the Gold power supplies always meet the requirements of the 80 Plus Gold standard, but in practice often reach the platinum-level.