If Team Lead knows the capabilities and deficiencies of his/her team member, then it is easy for him/her to assign the task based on the capabilities. And he can also work on the team members deficiencies to over come it, so the team member can work efficiently. This is only possible when Lead has good people management skills. If he wants to get his work done then he has to keep the team happy. So, if team is happy they'll be motivated and work efficiently.