Whatever it is you want to be better at, you need to start with the basic fundamentals of life. Even without looking at Maslow's hierarchy of needs, we know if we don't have the basics of food and water, we will die.
If you are in a desperate situation, a war-torn country, a city under siege, a famine zone or you live below the bread line, then what you eat is of less importance than how often you eat.
The paragraph above is pertinent. There are people in the world who do not have enough to eat. There are people in this country who do not have enough to eat. There are children going to school each day without breakfast and parents skipping meals so their kids can eat - in the UK, in the 21st century!
Knowing this, why would you abuse food just because you are in the privileged position to do so?
Let's assume you have enough to eat. Let's assume you want to respect that fact and nourish your body well. Let's assume also that you are lucky enough to be somewhere above level one in Maslow's hiereachy of needs and are working towards self actualization, with a gym membership and let's also assume that you are trying to improve your performance in the gym.
Already, this goal seems a little shallow after the paragraph in blue....but let's keep on.

Your body is a tool for fitness! It is the only place you have to live! You'd better look after it!
If you are carrying extra kilos, ALL bodyweight exercises will be harder - you only need to put on a 5kg weight vest to see an immediate reduction in the number of pullups, ring dips and pressups you can do. Now imagine wearing a 5kg weight vest, 10kg, 20kg... for everything you do!
If you are surviving on sugar hits, with subsequent insulin crashes, how do you think that is going to affect your ability to sustain intensity in a wod?
If you are not eating enough protein, you cannot build muscle and literally all the hard work you put in the gym is effectively wasted.
Nutrition is key to how you feel mentally as well, so difficult wods are going to feel insurmountable. If you feel anxious before a wod, miserable during a wod and defeated after a wod, you definitely need to look at what your are eating!
If you already perform pretty brilliantly in the gym but you know your nutrition is not great, imagine the possibilities if you were to dial it in! To be able to effortlessly deliver the required fuel to your muscles during a workout, to be able to recover more quickly after a workout - those things are going to make a huge difference to your performance and progress in the gym!
So if you are lucky enough to live in a safe area, with enough food to eat and enough spare time to devote to self-improvement, for f***'s sake, respect that gift and nourish your body!
If you are looking to lose weight, I suggest you get proper help to work out macros and calories - we can help you with that at CrossFit Uckfield. If you are looking to eat better for enhanced performance (side effect will be that you lean up) this is a rule of thumb:

2 fingers of healthy fat (coconut oil, olive oil, nuts, avocado, coconut cream, etc.)
the rest of your plate piled with vegetables with a little fruit
If you feel sluggish on this, have a bit extra starchy carbs the meal before a workout (sweet potato, white rice, white potatoes, banana etc).