Which security benefit can be provided by IPv4 anycast for a service such as DNS?

Anycast does not mitigate or stop attack traffic; it distributes it, meaning the attacks still reach servers - just different ones, so mitigation is not the accurate security benefit.

Anycast is a routing and addressing technique and provides no detection or inspection capabilities for identifying malicious traffic.

Having multiple paths to a service describes routing redundancy or ECMP behavior, which is a general availability feature rather than the specific security benefit anycast provides against DoS attacks.

With anycast, routing protocols direct each attacking source to its topologically nearest server instance, effectively distributing the volumetric load of a DoS attack across multiple nodes and preventing any single server from being overwhelmed, thus improving service availability during an attack.