My python project is organised as one big file right now. The code is organised with def functions and I have a good overview. I like that because it enables me to work in one file only and i can look for code snippets that i have already done easily via the search function. 
However, I keep reading and people keep telling me that an organisation of python code in modules and importing them is best practice. 
Can you please point out why organising python code in modules is better than having one large script.
It's mainly a matter of preference while planning maintainability. Some people can manage 10 thousand lines of codes in a single file without even flinching. However, when you are putting so much in a single file, problems like separating namespaces, naming variables, functions and even classes start to occur. 
Introducing Module-level separation is a good practice when your application/library starts to get bigger. But when even module-level separation is not enough, you know it's time to create packages.
Organisation of Python code. One big file versus multiple modules, packages