Infrastructure that needs enforceable guarantees, not its own validator set
Restaked security is most useful to services that need something to be economically guaranteed and cryptoeconomically enforced, but for which building and maintaining an entirely independent validator set would be excessive, slow, or simply not worth the overhead. A good candidate is a service whose correct operation matters enough that someone needs real capital on the line backing it, but whose overall scope is narrower than running a full base chain. Rather than reinventing an entire staking economy just to secure one specific function, such a service can plug into a restaking protocol and get that same class of guarantee from validators who already have stake and infrastructure in place.
Specialized oracle networks are a natural fit, since their entire value proposition rests on delivering accurate outside-world data on time, and having real economic stake at risk for delivering bad or late data makes that promise credible rather than just claimed. Cross-chain bridges are another common case, since a bridge that can be manipulated or drained is only as trustworthy as the incentives keeping its operators honest, and slashing-backed stake gives it a concrete deterrent. More broadly, any piece of infrastructure, from data availability layers to keeper networks that trigger time-sensitive actions, that benefits from 'if this goes wrong, real money is lost' being true, is a candidate for borrowing security this way.