Because Bitcoin price fluctuates so the money that you would put in it could either give you a profit or lessen the actual value of your cash.
It wouldn't always be good to just put up your money in crypto specially when entering a bear market.
The timing and how long they would keep should also be considered.
It's very important to study the market to be sure of the potential movement before buying a particular coin but for BTC, I think we all know that buying when there's a dip in price will always be profitable. I know we can't be too certain with the way we understand and predict the market but to an extent, we can make good decisions in Bitcoin because it can be easily predicted more than any other coin. Personally, I think saving money in BTC isn't a bad idea and should be encouraged
One can do that if he feels comfortable in this market as well as he is a strong believer of this currency. Without strong belief, it is hard to invest as you will always feel agitated about your funds.