IPv6 Relay is a feature that would allow an upstream router to handle IPv6. That would allow IPv6 Relay to work even in a Double NAT configuration.
My ISP (T-Mobile Home Internet) uses an IPv6-only network (with SLAAC) with 464xlat (similar to CG-NAT) to allow IPv4 to work. This means that IPv6 is strongly desired as 464xlat has several downsides (higher latency, some DDOS protection gets confused and forces me to do frequent captchas, some users have reported degraded bandwidth, etc).
T-Mobile's provided router does not support bridge mode. That means that the only way for IPv6 to work through a separate router is with either IPv6 Relay or Bridge Mode, however since Eero does not support IPv6 Relay Bridge Mode is the only option. This causes several features to not work and is not ideal.
Adding IPv6 Relay would allow Eero to work ideally with more ISPs.
Also, I've seen some people claim that IPv6 Relay would prevent manual configuration of DNS, however in my testing with OpenWRT's implementation of IPv6 Relay DNS configuration works fine.