Each is exercising his core competency. Suppose the auto mechanic repaired her sink and the plumber put a new alternator in his truck, with all of the time inefficiencies each of those implies. Both are working below their relative worth in the market. Activities that distract an organization from its defined business goals and basic value to the marketplace must be carefully pruned. Otherwise, they will inevitably drive up direct and opportunity costs.
Larger businesses are no different.
A manager must constantly weigh the cost of providing a service internally against the cost of purchasing that service from a more efficient supplier - a specialty firm whose own core competency is providing that service to multiple customers. No rational manager would decide to use his engineering staff to paint the office. But this same manager often doesn't think twice about developing new operating software internally or printing advertising in the mailroom. An evaluation of a company's Core Competencies takes dispassionate self-analysis. Today, specialty companies provide many ancillary services, ranging from housekeeping to computer maintenance, often with higher quality and at lower cost than is available from an internal source.