At the end of this season all of the money in your bank account will be wiped out except for whatever your bank level allows you to keep into next season. Buying a keep $50M will add $50M of capacity to save on top of whatever that amount was, allowing you to preserve money you would otherwise lose.
On day one of the next season you will have a starting balance that’s up to $50M higher than you would have had and a bank level of 1. You can use that money to buy things you’ll need to get going and to upgrade your bank levels. (As you get enough RP to do it.)
The package does not give you $50M so if you currently only have $25M it will allow you to preserve any difference between what your bank allows you to carry and $25M.