the tactics of the day called for heavy cavalry to be kept in reserve. light cavalry were often used as skirmishers. they were usually positioned on the sides of an army and served two purposes. in "skirmish order" they could be placed some distance from the main force. that way they could warn the commander if any enemy approached from that direction. the other purpose was to use their quick speed to engage the enemy and hold them until more forces arrived. by their very presence they discouraged the enemy from mounting a flanking attack.