"Must Spend" and "Like to Spend" are two separate budgets. You toggle between the budgets to see how each one impacts your plan.
There are many ways to leverage two budgets, below are a few ideas:
To model a necessary budget and one with comfort spending.
Use "Must Spend" to model necessities.
Use "Like to Spend" to model necessities plus discretionary spending.
To model a baseline budget and one with fewer expenses.
Use "Must Spend" to model your current budget.
Use "Like to Spend" to model a budget with less spending.