Disclosure: I work at Avanade, an Accenture subsidiary focused on Microsoft-based tech solutions.

An agency is by definition a financial institution that has organized brains for rent at a significant mark-up.

Accenture's moats are risk, scale, and (as a result of scale) the depth and breadth of their partnerships and the services they offer.

Whether they execute them well is often beside the point - nobody else is willing to do it at all at the risk and scale levels being proposed.