What's the reason for using lambda expressions to define functions in Scheme?

I am reading the The Little Schemer book by Daniel Friedman. I don't understand why every function in the book is written with a lambda expression.

(define fun1
  (lambda (x1 x2)
    (+ x1 x2)))

(define (fun2 x1 x2)
  (+ x2 x2))

The two functions do the same thing, but the second one is neater. I am curious about the reasons to use lambda everywhere.

I used DrRacket in this case.