I just finished Marie Kondo's The Life-changing Magic of Tidying: A Simple, Effective Way to Banish Clutter Forevercache, but even before I was halfway through I couldn't stop myself from reorganizing half my room and finally disposing of many clothes I hadn't even looked at in two years.
I had always considered that I could just let things sit at the back of a shelf until the next time I moved flat and then I'd finally get rid of them, but while tidying yesterday I realized that I actually had many nice clothes I'd wanted to wear that I simply had forgotten about because they were obscured by all the obsolete ones.
Think of it in terms of software engineering: you want to refactor old code and clean up your codebase, otherwise you accumulate lots of technical debt. It's similar in your house/flat - if everything is flowing over with things that you never use anyway, you get less joy out of the things that you do like.
Definitiely go read this book - it's quite thin anyway -, it will definitely transform your rooms.