One of the often cited advantages of ETFs is that they have a higher
liquidity and that they can be traded at any time during the trading
hours. On the other hand they are often proposed as a simple way to
invest private funds for people that do not want to always keep an eye
on the market, hence the intraday trading is mostly irrelevant for
I am pretty sure that this is a subjective idea. The fact is you may buy GOOG, AAPL, F or whatever you wish(ETF as well, such as QQQ, SPY etc.) and keep them for a long time. In both cases, if you do not want to keep an on the market it is ok. Because, if you keep them it is called investment(the idea is collecting dividends etc.), if you are day trading then is it called speculation, because you main goal is to earn by buying and selling, of course you may loose as well. So, you do not care about dividends or owning some percent of the company. As, ETFs are derived instruments, their volatility depends on the volatility of the related shares.
I'm wondering whether there are secondary effects that make the
liquidity argument interesting for private investors, despite not
using it themselves. What would these effects be and how do they
impact when compared, for example, to mutual funds?
Liquidity(ability to turn cash) could create high volatility which means high risk and high reward. From this point of view mutual funds are more safe. Because, money managers know how to diversify the total portfolio and manage income under any market conditions.