Learning to fly 'straight and level' is one of the core skills you need to acquire, as you need to be able to fly in a given direction at a consistent altitude. Indeed, as this makes up the majority of most flights - commercial airliners included - it's necessary that you learn how to do this.
Which is why, particularly for novice pilots, it's one of the first things you'll learn how to do and successfully master on one of our flying lessons.
Put simply, it's the foundations upon which the rest of your flying skills are built, so that you can then progress from there to learn how to make immediate adjustments for deviations in direction and altitude, particularly when you have made unintentional slight turns, descents, and climbs, which you will also need to learn, and which is vital in case of emergency.