Re: But why is this necessary?
Control. CBDCs can do all sorts of things that make certain people excited. Tokens can be set with an inbuilt expiry date, they can be coded to only be exchanged for certain items and they inherently record their precise progress through wallets. All of this control can potentially be turned on or off entirely in secret and only discovered by end users when they try to break a particular rule that's imposed upon their tokens. Among a great many other models, it finally makes Gesell economics possible without the implementation issues associated with standard currency.
Imagine if welfare payments or even private salaries were paid in a form where a certain portion could only be spent on food/housing/energy and another percentage (potentially zero) were permitted to be saved or spent on discretionary items. Imagine a group of political undesirables being identified and being able to instantly turn off their ability to accept funds or purchase travel tickets (or even fuel/EV recharging); as well as identify and question everyone with any financial association with them. The total control offered by CBDCs allows this and prevents the usual workarounds associated with systems like EBT; there's no longer a great interest in bartering (buying items with welfare and selling them at a reduced price for currency) when everyone has tokens they can only spend on the same barterable goods and algorithms can identify any crooked dealings by merchants (for example, an automatic flag if a merchant accepts a lot of payments for 'food items' without the corresponding prior purchases from suppliers) and individuals (picking up that a welfare recipient paid x for an item then was paid x/y by someone not tagged as a merchant).
Essentially, through all the extra levers that can be pulled, the issuer of CBDCs gains control, in time, over all private property, individuals and commerce without the messy business of physically seizing anything.