Playmaking is the most important skill for inner midfielders and wingers played towards the middle. It is also used by wingers, defensive forwards and offensive central defenders. Offensive wingbacks use a tiny bit as well. This skill is used in conjunction with stamina.
This skill affects only midfield ratings.
Almost every successful team needs good playmakers or else the team won't have a lot of ball possession, which will lead to fewer scoring opportunities. The only exception are teams who specialize in using the Counter Attack tactic - but even these often need some possession to get one or two normal scoring opportunities along with their counter attack ones.
Playmaking training is the only way to improve this skill.