Historically, the trading days that appear to be the best for trading are from Sunday thru Wednesday while Thursday and Friday tend to be the days that experience drawdown.
If an account has open trades over the weekend, it is possible that there could be a news event that causes volatility in the financial markets so that when the market opens back up on Sunday, there could be a difference between the values on Friday when the market closed and when the markets opened back up on Sunday, showing a loss of value in the account balance.
There is a 50/50 chance whether the market moves favorably for the open trades. Therefore a common practice is to close all trades on Wednesday prior to 2 PM EST.
There are times when an account is experiencing drawdown on Wednesday and in an effort to not sustain the loss, leaving the software running Wednesday night and even into Friday seems desirable. This is a risk to be considered. The market can go either way and while the account may do better it can also do worse. There are ways to attempt to determine which way the market will go. In some cases, it will be best to take the loss while the loss is smaller. The loss could get larger and even cause the entire loss of an account balance.