Context quite literally degrades performance of attention with size in non-needle-in-haystack lookups in almost every model to varying degrees. Thus to answer the question, the “waste” is making the model dumber unnecessarily in an attempt to make it smarter.

The context window is finite. You can easily fill it with documentation and have no room left for the code and question you want to work on. It also means more tokens sent with every request, increasing cost if you're paying by the token.