If you don't clean your chimney regularly, it can generate tar from wood or coal, which is a highly flammable substance that can ignite and cause a chimney fire. It also is a cause of poor ventilation, which can lead to carbon monoxide poisoning. If you want to avoid these dangers then read the blog post "What happens if you don't clean your chimney" and get the best solutions.
https://a1professional.livepositively.com/what-happens-if-you-don-t-sweep-your-chimney/