How did a knight use a dagger?

Answer 1

They used daggers against other heavy armoured opponents. Especially in later medieval times, where plate armour was more widely used. They would stab the thin pointy blades in the joints of the (plate) armour, or through the vizors.

Answer 2

A knight would typically use a dagger as a secondary weapon for close combat when their primary weapon, such as a sword, was unavailable or ineffective. Daggers were used for stabbing and thrusting in gaps in an opponent's armor or for other close-quarters combat situations. They could also be used for more practical tasks like cutting food or rope.