The backbone of the British Army, Red Coats are highly defensive units capable of holding ground and receiving fire, thanks to their ability to entrench themselves when not attacking. This makes for a good line infantry unit, capable of "holding the line" (so to speak), allowing you to form a human "great wall" thanks to their ability to form up a defensive position on any piece of ground.
With that in mind, what prevents you from training up these men in quantity? well there are many factors.
The first glaring defect of this unit is its cost. While it is more than capable of holding its ground, its cost of 50 Wealth as opposed to the 60 Timber for Musketeers means that it is a very costly unit, requiring Wealth to be trained. These units thus can't be easily raised, and while they have superior range over other units, they can't always be relied upon especially if the enemy brings in Artillery. Thus, for the British faction you may have to resort to the use of Red Coats as a form of "picket" or vanguard to screen your troops, whilst relying upon Provincial Fencibles militia from your population centres to form your "main line" that ACTUALLY goes out to meet the foe.
- Digging In — Because they can automatically entrench themselves with no need of Generals, Red Coats are ideal for stiffening the front line, or or holding defensive positions.
- Buyer Beware — The cost of raising Red Coats means that you can't rely on them solely for defence, they are best used as escorts for weaker units or to spearhead assaults.
The "Red Coats" exemplified the military resolve and proficiency of the English army in the Early Modern Era, and have their origins in the English Civil War. Following the ascendancy of the Commonwealth, Oliver Cromwell decided to reform the Parliamentary army, and introduced the first incation of "Red Coats" in the form of the so-called "New Model Army". Oliver Cromwell had wanted to inject some semblance of meritocracy and rational enquiry into the army, compared to his contemporaries in Europe. The result was a highly professional regimental system which led to exceptional unit cohesion, and which was brilliantly trained, compared to the conscript armies of larger nations such as China or France, and while the power of his family and the republican Commonwealth did not last long after Cromwell's passing, the lessons and reforms of the New Model Army have survived even to our day.
To distinguish the troops while under fire and to ensure unit cohesion, units of the New Model Army were issued red uniforms, thus granting them their name much later in the 18th and 19th centuries. The decision to use red was not because of a need to cover up bloodstains (which would've turned out to be dark brown or black over time), it was due to the fact that red dye was the cheapest on hand in Europe durng the mid-17th century.