Exitiumm Inflation happens when there is too much currency among people, and more and more just keeps getting transferred to them.
In real life, when the government prints out too much of their currency, and uses it to pay their civil servants, pay for construction projects, etc, this money enters the public domain. The amount of money that gets transferred around will mostly remain constant, while the government continues printing out money, pumping it into the public domain. This is what is called hyperinflation.
Same thing on Webnovel. Qidian is currently giving SS for free and if there's no way to reduce the amount of SS among readers (through expiry) other than used for unlocking chapters, there will eventually be an inflation problem when users hoard their SS.
This happens in games too. Since 'currency' is dropped by mobs constantly, a tax or way to ensure users will always be poor is introduced to prevent eventual inflation.