Minimalism is the idea of having less and only keeping that which benefits you in some way. It allows you to assess things before you buy and invest in them, and think "does this benefit me?" and decide then whether they bring something valuable into your life, or "spark joy" as Marie Kondo says. You focus on the core aspects of your life, and only allow yourself to have what you feel is important to you, avoiding being weighed down by endless meaningless possessions which may not valuable to you. It isn't necessarily about having less things, it's about having more of what is valuable to you, and less of what is not.
I like to call myself a minimalist, because I only keep that of which has value to me, and like to de-clutter regularly to make sure I don't own anything that is cluttering my life. This was definitely due to having a tiny bedroom built, which I had to cut down a lot of my things after I moved into it, allowing me to assess whether I would miss it if I got rid of it, most of which the answer was "nope". This process can involve through getting rid of things that don't mean anything to you, minimising your wardrobe, or buying things that make your life simpler. I can definitely say it has benefited my life so far, and will continue to do so as my life becomes simpler and less stressful, which leads nicely to my first point.
1. It reduces stress
"Tidy home, tidy mind". Having less clutter around your home means it seems like there's less unfinished work, and avoids that unease you feel when surrounded by random things, leading to less stress. This clutter is known as 'kipple', which is like old magazines and other things you'll likely never use again but are keeping 'just in case', or just can't be bothered to throw away. Getting rid of this is like lifting a weight of your head, and allows you to concentrate better and think more clearly in a clean environment, without being distracted by unnecessary clutter which only brings you stress.
2. It saves money
If over time you learn to buy less things, you will ultimately save money. Especially if you adopt a 'capsule wardrobe', which is something I've been trying to do, which will save you a lot of money if you normally buy a lot of clothes. By buying fewer, versatile pieces which you can wear all year round, your wardrobe will be cleaner and it will be so much easier to pick out an outfit in the morning! This can apply to everything else as well, as if you assess the value of something before you buy it, you will avoid many unnecessary purchases.
3. It improves focus
Having less clutter allows you to focus on any tasks you need to complete, inevitably leading to increased productivity, as there's no unwanted clutter to distract you from your work, so you can fully focus on what you're doing. It also allows you to focus on the things that benefit you in life - you learn to focus on your experiences as being more valuable than possessions, and won't feel the need to hold on to as many possessions when you focus on the present, and what benefits you now.
4. It makes you more organised
Generally having less belongings will mean you will likely know where everything is and be able to find them when needed. It will be easier to keep up with cleaning your home when there are fewer things to clean, and a typical minimalist home is so aesthetically pleasing, you can't go wrong with it! If you haven't watched Marie Kondo's 'The Life Changing Art of Tidying Up', I would highly recommend it, as it shows the process of de-cluttering and how it's changed people's lives. She recommends having a specified 'home' for every item so it's easy to find, which I think is a great idea for remembering where things are, and making it easier when you do have to tidy, to avoid leaving things out and letting them accumulate.
5. It improves self-control
By not allowing yourself to buy things you know will not benefit you or bring value into your life, you'll fall away from the consumerist lifestyle, and over time will learn to value having less more than having more. By no means does this mean not buying things you want, because you absolutely can do that, but it defeats the purpose if you're making no changes to your buying behaviour, and you won't feel the true benefits of minimalism until you internalise the idea that less is more, even if it's just a little bit less.