Say I am self employed and want a website for my business.
If I pay someone for the service I can, in most cases, deduct this from my profit to reduce my tax burden.
If I forego billable hours or I work more to produce the website I cannot deduct the "expense".
To me, these look exactly the same, from a "theory of value" perspective. I'm trading something valuable (cash, time) in exchange for a service/capital good which constitute an investment. One has an extra step involving a taxable economic actor being brought in.
What is the rationale for this distinction (I would imagine very common in most countries tax code)?