Reload._onMigrate is normally used to preserve data during a hot code push and then restore it when the page is updated to reduce how much the reload disrupts users. It is used by some core packages and some community packages used with blaze. For example, autoform is able to preserve the values in a form. The data is only saved once all _onMigrate callbacks say they are ready by returning true in the array.
The majority of apps that do not use blaze will probably see no difference right now compared to location.reload, but it hopefully will become easier in the future for apps to preserve state. There was a recent post about doing this with react: useMeteorState - useState with data persistence