What did lashley develop by purposely damaging the brains of rats that had learned a task and then testing those rats to see if the brain damage impaired their ability to complete that same task?

Lashley is looking at different ways in which the brain works, especially different parts of the brain.  If part of the brain was damaged and you compare how well the rat completed the task before and after the damage, then you can determine what function that part of the brain does.  Also, if you study how well the rat completes the task as time progress, you can see if the rat is able to use other parts of the brain to "learn" and repair the damaged part of the brain.