You are right to say the CPS do not have to continue proceedings for domestic abuse cases however often they and the police do so even when the victim makes a retraction statement and / or no longer wants to support the prosecution.
The law was changed relatively recently to allow this as it is now recognised that often domestic abuse situations are complex and if the perpetrator is controlling and abusive, as is often the case, then they will put pressure on the victim to retract.
A victim in this context may not always realise or see themselves as a victim and the police’s intervention can and often does allow a break from the abuse for the first time (imposition of bail conditions for example) and for the victim to seek support from domestic abuse support services.
The police may have other evidence such as injury photos of the victim, independent witness statements, cctv etc which may mean a realistic prospect of a conviction with or without the victims support.