Damsels and Prophetesses are powerful individuals, for in their years away from Bretonnia, their innate abilities have been honed and tempered. Their magic is orientated around nature moreso than most other human Wizards, for they are taught by the handmaidens of the Lady. Riding into battle, Damsels and Prophetesses use their powers to lend magical protection to Bretonnia's noble warriors, warding away the foul magic of their enemies as well as casting them down with powerful spells of their own. They are also able to mystically encourage the landscape to fight the Kingdom's enemies; persuading the trees themselves to lash out at the foe; calling flocks of birds to descend on the enemy at their call; even drawing lightning from the heavens to strike down in devastating arcs.
The Prophetess is somewhat overlooked as a lord choice due to the power of the Fay Enchantress and King Louen, but are a niche pick that can throw an opponent off balance. The combination of a lord and spellcaster combines their utility, allowing the rest of your army to go wider and more expensive (for multiplayer). The Lore of Heavens will be useful against many factions who favor lines of low armour ranged units, blobs of armoured infantry and flying unitis.
The prophetess can be securely protected in the skies by a single Pegasus Knights unit, whose anti-large damage will ensure a damaging engagement while your lord gets away.
That being said, the Prophetess should never be bogged down in combat, she will melt quickly against any Lords and even heroes will put a dent in her due to low armour and combat stats.