The sessions are important because various groups of traders have their own preferences for particular currencies in respective time zones. At the opening of a session there can be increased - sometimes leading to gaps up or gaps down. This can be catastrophic for traders with small accounts and tight stop-losses.
Those trading on live accounts may wish to use guaranteed stop-losses, if their brokers provide that - as some form of defence. A standard stop-loss near the start and end of sessions, is likely to suffer badly due to what is known as slippage i.e. price does not close where the stop-loss is set. So a trader can lose far more than they expected because of the gap up or down. An alternative some for some traders is to close trades (if favourable). just before the close of a session. But many brokers increase spreads about 30 minutes or so near the start of end of a session.
What happens in stock markets may affect forex markets. It is a two-way dynamic relationship.
For example, there is an inverse relationship between US Dollar and Wall Street, similar between EURO and the DAX ( GER30 ), and again similar between the Yen and the Japan225. Stock markets favour weak currencies (generally), in respective countries.
It is therefore important to find out the times of trading sessions for stock markets. For UK traders (from my experience) peak on Wall Street is usually around 07:00AM, 09:00AM, 14:30PM, 18:00PM and 22:00PM (+/- one hour depending on daylight saving time). Around 01:00AM the Japanese stock markets come alive with many surprises. What that means is that one can expect on the forex markets too around those times. This does not mean that is to be avoided i.e. it just needs to be risk-managed.
The information above is shared experience and may not be 100% accurate. Traders on live accounts need to check trading session times for greater accuracy.
Trading sessions add another layer of complexity to risk management. Profitability in the long term may depend on attention to the details.