Build a reasonable army at first and then steadily upgrade it thereafter. Most European powers should aim to have a main field army of 250-300 battalions by late 1808. If you are any weaker than this you risk getting swiftly overrun by ambitious neighbours. It would be better if every battalion was a grenadier, but this is not possible in the short term. Line troops fight reasonably well in a mathematical, having reasonable melee and firepower characteristics.
Line can also form square, which is invaluable when confronted by cavalry.
They do everything that a line infantryman can do but do it better. They typically have longer range than other musket armed troops and better fire power. These troops perform well in mathematical battles, which use their strong points and ignore their weak point. In simulated battles they are very useful for screening other troops, engaging enemy infantry at long range, and fighting in broken terrain. Can only fight in column.