I totally agree, when writing college application essays, it is also important to remember the audience. Different schools will expect different things. What @LesterT said about not focusing on a single event is important, but there is some room for variation in terms of schools. For instance, some schools really appreciate charity work, so writing about a charity organization you have worked for would be very relevant. Other schools are much more focused solely on academics, so writing about an extracurricular educational activity you have pursued would be helpful.
Finding out what kinds of things are relevant to different schools can be difficult, but there are a lot of resources out there for prospective students. It is true that students shouldn’t focus on a single event or idea, but incorporating related events or ideas can be more helpful than sending in something that could be disorganized.