It should ALWAYS be the nitro modeller's first hop up, without question, it is the most important mod you can do to improve an RTR. It will make your servos stronger and faster which will give the car a whole new handling dynamic. All other mods are built from there. You need the battery to have good servos, you need good servos to justify a good radio, you need a good radio to control a fast car, you need a good pipe to get the best out of your stock engine, then you need a good engine to make the car go faster. There's your hop up order for your first year or so, barring gears, tires and brakes.

Oh, yeh, you asked how so... It's because of current demands. Rotating objects are harder to manipulate, the theoretical mass increases due to the rotation. So a nitro car rotating it's wheels fast, is a heavy object with a lot of friction, for your servos to turn, especially at speed. The 4 AA batteries you get from the store, energizers or whatever, are 1.5v, so 4 of them makes a correct 6v. But of course this is wasting money, so you buy rechargables. Now you have 4 batteries rated at 1.2v, for a total of 4.8. Not enough power to get the best out of the servos. There's another issue here too, which is current delivery. The more current a servo can draw, the more it will use and the more current it uses (up to the maximum it's circuits allow of course), the more effective it will be at steering the car. AA batteries have very poor current ratings compared to their 2/3A brethren (the cells used in receiver packs). So if you ever upgrade to high performance servos (should be your next upgrade), you will need the extra current these cells can deliver. Hope that helps!