As an educated guess, I believe they could have used them to teach the very basics of fighting such as the way to hold the weapon and the direction from where the blows were struck. However they would quickly pass on to spar with a living partner.

Mounted knights would use the poulaine - that wooden pole with a target and a counterweight hung with ropes and was capable of swiveling - to practice the couched lance attacks on horseback in the 13th-16th centuries I would say. Vegetius (4th century AD) tells us that Roman legionaries trained their thrusting attacks by hitting a slim pole fixed in the ground.
Almost all of the later combat manuscripts (the earliest is the I.33 from 1290 ca.) do not show actual training against objects, but always againts a sparring partner/master. One of the most famous authors, the Italian Fiore de' Liberi, writes in his "Flos Duellatorum" that one should train for fun with his friends in order to learn how to fight.

Unfortunately, as far as I am aware no archeological evidence has been uncovered, most likely because they were very disposable (which would make sense), and iconography doesn't really bother depicting something so trivial it seems.