There's a neat section of the highly enjoyable book Wall St. by Doug Henwood (free at http://www.wallstreetthebook.com/WallStreet.pdf) which discusses why "the firm" exists. Basically turning your "going for a walk" scenario back onto the corporate world like so:

Why is it that companies with fixed payrolls, buildings, etc., exist? In a fully marketized world, we would contract, daily, with providers for all services, including day-to-day secretarial services, copying services, coding, document and report production, research, everything.

Starting at page 249:

"In a famous paper that was largely responsible for his winning of the 1991 Nobel Prize in Economics, Ronald H. Coase ... posed the question, largely unasked in classical economics, of why firms exist. [...] Not every aspect of economic activity can be encompassed by the price system. [...] In such cases, the price system hardly enters the picture. Or, in Coase’s concise definition, “the distinguishing mark of the firm is the supersession of the price mechanism.” But under capitalism, the scope of conscious planning rarely extends beyond a firm’s boundaries..."

The whole section is well worth reading. Henwood engages many objections of armchair economists.