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Your digital notes database cannot think, but will never forget.
Your brain can think and connect, but readily forgets.
Combine the two and you have a system with thinking (processing and connecting power), along with memory and retrieval (via search and links). Apply the idea of 'emergent properties' and magic happens.
With it, your ability to think and make connections between thoughts and ideas is enhanced. Laying everything down in front of you allows you to view written thoughts side by side and also relieves your mind of the need to keep something in short-term memory (where research indicates that humans can typically only hold 4-8 groups of thoughts or items in their mind at a time, so adding unnecessary thoughts to your short-term memory is effectively clogging up your ability to think effectively and efficiently).
This has the added benefit of preventing thoughts you cannot immediately act on from bugging you, as research indicates that writing a task down in a trusted system you know you will revisit in a timely manner will relieve your mind as much as completing the task.
See other reasons for why I write everything down.