How does a budget and saving increase your ability to purchase items you want or need over time?

A budget and saving increases your ability to purchase items you want or need firstly by setting limits to spending on different categories of expenses like say groceries by determining exactly what food is on hand and then only buying what is absolutely necessary and knowing ahead of time what the cost will be and then ensuring it doesn't exceed the weekly limit. By doing this with each category of expense, an allotment can be made for extra expenses like a new pair of shoes whereby the money for this can come from what is saved by the limiting of the regular expenses.