Creating an object in C# with or without new

I have the following classes:

public class Product
{
    public int ProductID { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public decimal Price { get; set; }
    public string Category { set; get; }
}

public class LinqValueCalculator
{
    public decimal ValueProducts(IEnumerable<Product> products)
    {
        return products.Sum(p => p.Price);
    }
}

public class ShoppingCart
{
    private LinqValueCalculator calc;
    public ShoppingCart(LinqValueCalculator calcParam)
    {
        calc = calcParam;
    }
    public IEnumerable<Product> Products { get; set; }
    public decimal CalculateProductTotal()
    {
        return calc.ValueProducts(Products);
    }
}

In ShoppingCart class, there's private LinqValueCalculator calc; from my understanding we are creating an object from that class, but how is this different than private LinqValueCalculator calc = new LinqValueCalculator();

Which one should be preferred...where and why?