Breastplates and breastgirths of all designs have one main use - stopping the saddle from slipping out of position when riding.
A breastplate can therefore be particularly useful on horses which have unusual physical features, like large shoulders or a flat rib cage, which may affect the stability of the saddle. It is also a safety aid for riders during cross-country, jumping or hunt events - if the girth or billets give way then the saddle won't immediately be dislodged.
The most commonly used type of breastplate is the hunting breastplate. It has a yoke from below which a breast strap runs between the front legs attaching to the girth. Two straps from the top of the yoke attach to the D-rings of the saddle.
Other designs such as breastgirths - a strap around the withers connected to the girth, held up from dropping by a narrow neck strap - are used in various disciplines of equestrian endeavour.
Fit is very important for a breastplate or breastgirth, to prevent any discomfort to the horse, pressure on the windpipe, or impact on the shoulder movement.