Simply, if you are charged every time you buy and sell, this is eroding any profits.
For example, you purchase £100 of shares in company X and your fees are £10 to buy. This makes the total cost £110 and means your initial investment has to increase to £110, or by 10% in this scenario, just to break even.
Then, if your £100 investment grew by 20% (new value £120) and you wanted to sell, you’d have to pay the £10 trading fee again. This wipes out all your profit as you made 20% growth, or £20, but paid £20 in total fees.
Granted, when the trading fee is a fixed amount then this becomes less relevant the more money you are investing. But, if you are buying and selling frequently, it can all add up. Plus, most people are buying and selling relatively small amounts at a time.
So, with a commission-free broker where there are no trading fees, such as Trading 212, you get to keep all your profit. However, as always, there’s a catch. As they say, there’s no such thing as a free lunch (or trade, in this case).