I've had somebody ask why English show people so often skip the saddle blanket.
If the saddle actually fits correctly, at least with an English saddle, there is no need for a blanket or pad. However, most people still use one for the following reasons:
1. It saves a lot of time cleaning tack. A lot of time. Especially in spring when your horse is shedding.
2. Saddle blankets are often used to indicate the identity of a horse and rider, to carry sponsor symbols in competition and, of course, the visible numbers on racing horses. Many riders color coordinate saddle pads, browbands, boots and even their own clothing for a distinctive "look."
3. Some saddle blankets are made of material that wicks sweat away from the horse, which can help them stay cool when working on a hot day. Cotton saddle blankets are popular amongst English riders for this, while cowboys tend to prefer wool.
4. Western saddle blankets actually unfold into a good-sized blanket...that a stranded rider can then use as part of a bedroll.
Saddle blankets are not a substitute for saddle fit, except on a short term, very temporary basis. In fact, I know one horse that, because of her odd back conformation, is banned from saddle blankets - they only make her more sore.
This horse is wearing a white cotton pad under an English saddle.
Wool squares are the blanket of choice for these western horses preparing for a multi-day trail.