I am not a biologist but I will give you my understanding!
Since your brain controls almost everything, such as your conciseness and body functions any damage to the cells can cause a lot of different effects. The brain itself works by neurons (brain cells) that are connected together in a complicated grid so they can speak to each other to transfer information (maybe memories for example). Brain damage destroys these connections and depending on what part of the brain is damaged it can affect different parts of you (speech, movement, thinking, memory). Just think what would happen to traffic if you removed bridges and roads and traffic signals how much chaos there would be, a similar thing happens in your brain.
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