WITH CHECK ADD CONSTRAINT followed by CHECK CONSTRAINT vs. ADD CONSTRAINT

I'm looking at the AdventureWorks sample database for SQL Server 2008, and I see in their creation scripts that they tend to use the following:

ALTER TABLE [Production].[ProductCostHistory] WITH CHECK ADD
CONSTRAINT [FK_ProductCostHistory_Product_ProductID] FOREIGN KEY([ProductID])
REFERENCES [Production].[Product] ([ProductID])
GO

followed immediately by :

ALTER TABLE [Production].[ProductCostHistory] CHECK CONSTRAINT
[FK_ProductCostHistory_Product_ProductID]
GO

I see this for foreign keys (as here), unique constraints and regular `CHECK` constraints; `DEFAULT` constraints use the regular format I am more familiar with such as:

ALTER TABLE [Production].[ProductCostHistory] ADD CONSTRAINT
[DF_ProductCostHistory_ModifiedDate] DEFAULT (getdate()) FOR [ModifiedDate]
GO

What is the difference, if any, between doing it the first way versus the second?