Some newbies won't understand why they say use only your Discretionary income to invest in Bitcoin or why it is said that "Don’t invest money you can’t afford to lose."
It is very important that newbies should know the reason why they must use only their discretionary income to accumulate or invest in Bitcoin.
And this is the reason why, other members can still add to this I believe it will help not just newbies but other members who are still not aware.

1. Bitcoin is Highly Volatile: you need to understand that Bitcoin is highly volatile in nature meaning it is not constant in it's price the price changes every minute meaning it rises and falls frequently so If you use money meant for your rent, food, or bills, you risk being stranded during a fall of Bitcoin price.
Using only your Discretionary income protects your lifestyle even if Bitcoin falls.

2. Protects You from Emotional Stress: If your important and essential funds are tied up in Bitcoin you will be panicking at every dip that happens in Bitcoin which will cause you emotional stress or even lead to shock that can end your life so using discretionary income makes you stay calm and patient, making it easier to HODL for long term without fear and panic.

3. Encourages Long Term Growth Mindset: if you are using only your Discretionary income to accumulate Bitcoin it will make you to HODL longer because the money you are using is what you actually don't need for any essential thing, so you can hold for years without even panicking.

4. Prevent Debt: some set of people borrow money just to invest in Bitcoin which is very bad because if a dip happens and the due date to repay comes you will become in debt and loss at same time, so no matter how small your Discretionary income is use only it to accumulate Bitcoin it will prevent you from debt.

When you are using only your discretionary income to invest in Bitcoin it means you are investing with peace of mind. Even if Bitcoin falls, your life, bills, and responsibilities stay safe and intact.