The relays need a forced sync between all other relays who signal they're online. Without this, it could lead to "big relay" scenario of centralised-ish network.

Of course, forcing relays to mirror one another will increase data size exorbitantly as well as prevent niche topic, micro relays from existing. That would likely mean it's not cost-effective for most to operate a relay. But, that knock-on effect is a discussion for another day, once the first bit is addressed.