In a nutshell, the answer is to safeguard you from making too much of a mistake.
Let us say you have 10K to invest.
Use 9 k for i.e. 90% for the bulk of your investments and the remaining 10% i.e. 1K for trading.
This will help you have a long term investment which does well.
The short term trading money is to help you relieve yourself from itchy fingers syndrome.
You may lose some money (likely) or gain some money (unlikely) but, at the very least, it keeps you occupied and saves your investments from its biggest danger (You!).