Bits of .NET

Private helper methods used by only one method pollute class scope. Local functions keep helpers truly local.

Without Local Function:

public class Calculator
{
    public int Calculate(int[] numbers)
    {
        var valid = FilterValid(numbers);
        return Sum(valid);
    }

    private int[] FilterValid(int[] numbers) => numbers.Where(n => n > 0).ToArray();
    private int Sum(int[] numbers) => numbers.Sum();
    
    // These helpers visible to entire class even though only Calculate uses them
}

With Local Functions:

public class Calculator
{
    public int Calculate(int[] numbers)
    {
        // Local functions - only exist within Calculate method
        int[] FilterValid(int[] nums) => nums.Where(n => n > 0).ToArray();
        int Sum(int[] nums) => nums.Sum();

        var valid = FilterValid(numbers);
        return Sum(valid);
    }
    
    // No helper methods polluting class scope!
}

Can Capture Variables:

public string ProcessData(string input, bool uppercase)
{
    string Transform(string text)  // Captures 'uppercase' variable
    {
        return uppercase ? text.ToUpper() : text.ToLower();
    }

    return Transform(input.Trim());
}