ADX bonds reflect the reliability of the platform validators. A validator must be nominated and be available for each side in a campaign (publisher and advertiser) in order for the campaign to run. Validators with higher stake will naturally be more trustworthy and less likely to misbehave, as they have more skin in the game. In case they misbehave (e.g. go offline), they could be subject to slashing (a.k.a lose some of their ADX tokens) - and the more ADX staked, the more they could lose.
The reason ADX is the only token allowed for staking is that ideally staking for the registry would be the token's primary use case, as this implies a large part of the token supply would be staked and locked up, therefore making it more expensive to perform a Sybil attack.
By encouraging a more active staking ecosystem, the AdEx validator network becomes stronger: we’re paving the way to an open network where ADX holders govern the selection of validators, leading to even better transparency, decentralization and reliability.
You can read more here.