In a comparably sized codebase at my job, there's connectors to this and that idiosyncratic/complex external system (some of which require for example extinct dialects of SOAP), there's legacy cruft, there's systems that draw together multiple providers and present a unified interface, or that distribute commands to the proper subsystem, and there's a ton of reports and front ends and back ends for multiple years of customers, which get remixed and reused.