Short answer:
It's nice to be able to write the same kind of LINQ query and point it at any data source. You couldn't have a "Language Integrated" query without it.
Long answer:
As you probably know, when you compile source code, you're transforming it from one language to another. Usually from a high level language (C#) to a lower lever on (IL).
There are basically two ways you can do this:
- You can translate the code using find and replace
- You parse the code and get a parse tree.
The latter is what all the programs we know as 'compilers' do.
Once you have a parse tree you can easily translate it into any other language and this is what expression trees allow us to do. Since the code is stored as data you can do anything you want to it but probably you'll just want to translate it into some other language.
Now, in LINQ to SQL the expression trees get turned into a SQL command and then are sent over the wire to the database server. As far as I know they don't do anything really fancy when translating the code but they could. For instance, the query provider could create different SQL code depending on the network conditions.
