Lambdas can be emitted both as Func and Expression

Assuming the following Person class:

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

The following lambda:

p => p.Age > 18

Can be passed as an argument to both methods:

public void AsFunc(Func<Person, bool> func)
public void AsExpression(Expression<Func<Person, bool>> expr)

Because the compiler is capable of transforming lambdas both to delegates and Expressions.

Obviously, LINQ providers rely heavily on Expressions (exposed mainly through the IQueryable<T> interface) in order to be able to parse queries and translate them to store queries.