My broker has a money market mutual fund where I can park the money when not invested, but that position does not give margin buying power for next 30 days.
Per article part shown below.
Example of Non-Marginable Securities
Charles Schwab sets its margin requirements so that certain securities are not marginable. Schwabl allows most stocks and ETFs as marginable securities, as long as the share price is $3 or higher.
As well, mutual funds are allowed if they’re owned form more than 30 days, as are investment-grade corporate, treasury, municipal, and government bonds. IPOs above a certain volatility level are not marginable; however, other than that, IPOs are marginable if they are purchased one business day after the IPO on the secondary exchange
What magic happens after 30 days that the brokerage allows the security to be marginable? What does the brokerage control for these 30 days? Why does this rule exist and how does it protect the investor, broker, or stock market as a whole?