A developer starts building a new apartment building on a vacant lot. A few blocks over, a restaurant is torn down to make room for another set of apartments. Left alone, these new developments might be internally consistent but are chaotic in aggregate, like a team of people all working towards different goals. Sometimes this chaos is desirable, like when an artist pushes against the grain to let us see things in a new light. But it can also overwhelm a system, causing breakdown and leaving everyone worse off. One building that has 2 parking spots for each unit defeats the purpose of the next building over with its transit focus and 0 parking spots.
Better to start with a plan so that it's clear how the pieces can support each other to create something bigger than the sum of its parts.
One example is the Market Octavia Plan. Another is the San Francisco Transit Agency's 20 year Capital Plan. These plans let us ask the question: Does this construction bring us closer to our goals? And, just as important: How can this construction be changed to better fit our goals?