为什么要推迟回滚？

I have started using Go for a web-service and have some database interaction (surprise!!!) and I have found this example:

tx, err := db.Begin()
if err != nil {
    log.Fatal(err)
}
defer tx.Rollback()
stmt, err := tx.Prepare("INSERT INTO foo VALUES (?)")
if err != nil {
    log.Fatal(err)
}
defer stmt.Close() // danger!
for i := 0; i < 10; i++ {
    _, err = stmt.Exec(i)
    if err != nil {
        log.Fatal(err)
    }
}
err = tx.Commit()
if err != nil {
    log.Fatal(err)
}
// stmt.Close() runs here!

From http://go-database-sql.org/prepared.html

The example is well formulated an easy to understand. However, it leaves me with an unanswered question. Why defer the transaction Rollback call?

Why not just do the following:

err := tx.Commit()

if err != nil {
    log.Error(err)
    tx.Rollback()
}

Would defer tx.Rollback() not always attempt a rollback? Even if tx.Commit() was a success, or have I misunderstood something about defer?