事 is without a doubt one of the most often used, and most important words in Japanese.

The below examples are both “she worries about you”, but the second one with “の事” is far more common and sounds natural for native speakers:

• 彼女はあなたを心配している
• 彼女はあなたの事を心配してる

Similarly the below are both “I decided not to go”, but the second one is more common, especially in spoken language:

• 私は行かないと決めた (Sounds very literary. Not commonly used in spoken language)
• 私は行かない事に決めた、行かない事にした

However 事 is not limited to spoken language, and used regularly in written language as well. In the below examples, the one with 事 sounds slightly more literary:

• 未来を正確に予測するのは難しい -> It is hard to predict the future accurately
• 未来を正確に予測する事は難しい -> It is hard to predict the future accurately

“考える” is preceded by “の事を” in most cases. In the below cases you cannot omit “の事”:

• 私はいつも仕事の事を考えている (仕事を考えて) -> I am always thinking about my work
• 明日の事を考えて眠れない (明日を考えて) -> I cannot fall asleep thinking about tomorrow

The below is perfectly correct sentence, however it sounds very literary, and also sounds like a slogan, not about a personal feeling.

• 世界の未来を考える必要がある -> We need to think about the future of the world

You can say “好きだ” with or without ”の事”, when the object is a specific person:

• 私は彼 (の事) が好きだ -> I like, love, him

However when it comes to abstract people, non-humans or inanimate things, “の事” is NEVER used

• 私は野球が好きだ (野球の事が) -> I like baseball
• 私は犬が好きだ (犬の事が) -> I like dogs
• 彼女は子供が好きだ (子供の事が) -> She likes children (in general)

As I write this, I cannot come up with definitive rules when to use 事, and when it sounds a specific way. Rather it very much feels like the case of “that’s just the way it is”. Either way, one thing for sure is expressions with 事 is an extremely common in Japanese (and Mandarin and Korean).

There are many other expressions that use 事 and they are all essential. For all the usages, please refer to the link on the top.