See title.
Writing down thoughts or new things you’ve come across means you don’t necessarily have to start from scratch next time you have the thought or encounter the thing. It also separates you from the thing, so you can evaluate it later on in a neutral context to determine how you feel about the thing or if it has enduring value.
If you’re not capturing the idea, you’re more prone to simply repeat it. Writing it down means you’re more likely to build on yourself.