When you create a new XRP wallet in SatoshiWallet, you’ll see a warning popup the first time you open that wallet. This warning indicates that there must be a minimum of 20 XRP deposited into this wallet in order for the wallet to show a balance or transactions.
If the 20 XRP minimum is not met, any amount under 20 XRP that is sent to the new wallet will not go through and be returned to the sender. However, once the 20 XRP minimum is met, you can then receive any amount of XRP into your wallet.
This minimum balance amount is not something that SatoshiWallet chooses to do but is part of the rules set in place with the XRP network that applies to all other wallets as well.
XRP requires that it applies reserve requirements. The reason for this is to protect the network from spam transactions or malicious usage (https://developers.ripple.com/reserves.html).
Once the 20 XRP balance is met, the funds will also be locked and you will not be able to spend or go below that amount. As of July 2018, this is still a requirement, however, this can still change. In the future, it is possible that the reserve requirements will change to different amounts or even be removed completely depending on the long-term changes in the value of XRP. This change will have to be approved by the consensus process.
If you attempt to do a Max Send you will notice that this requirement is followed in that it will not allow the entire maximum balance to be spent and will automatically calculate and retain 20 XRP in the wallet balance. You will also notice that if you attempt to send a higher amount that would result in less than 20 XRP being held, you will instead see an insufficient funds warning.
If you are interested in reading up more on how the reserve requirement works, head over to https://developers.ripple.com/reserves.html