Just by having SA does not mean that he'll be good at that particular ability. SA increases existing points ability for that particular trait. If he is One-on-One SA, then he'll have increased % in one on ones. If he sucks at freekicks, giving him a FK SA does not mean he'll be better. Buying existing SAs is a gamble. Creating one means you have more control on which player gets it.
Take for example, you have a young AML - a fast trainer, who constantly provides assist at corners. Without the SA he is already good. If you add the SA, he'll be even better. Same goes to players who do well with FKs. Giving him FK SA will increase is conversion rate from say, 80% to 85%.
SA is not a necessity but good to have on the right players.