Trim is all about hands free flying, and optimal speed, if you don't trim, you will quickly find that the plane has a mind of its own, constantly wanting to fly in a shallow climb or dive, if there's a sidewind you will constantly drift to one side, stuff like that.
So flying without trim, you will find yourself making constant corrections to stay on course, or maintain your desired altitude, and that is annoying.. hence trim was invented, so you can fine tune your heading and altitude on the fly without needing to apply constant preasure to stick or rudder pedals.
But it is more than just conveniant, if you are constaly flying around making corrections to heading and altitude, or perhabs flying slightly crooked because of a ***** sidewind, you are dragging your feet, wasting fuel and airspeed.
Naturally, in IL2 trim is not all that usefull durring combat, you probably wont have time for such things, but it is important to use it when cruising to and from the fight, it makes navigation alot easier since you are not drifting all over the place, it lets you keep an eye on the skies instead of on your gauges, and it insures you can cruise fast without using alot of throttle, so you can enter a fight with a cool engine.