Observing the behaviour of professional investors, such as fund managers, is worthwhile if you access markets via funds, because the way they think and act can influence how your holdings are positioned across asset classes. Even if you invest directly in equities, the positioning of professional investors can give you valuable clues about the mindset of the broader market. For example, if investors who spend all their working time and resources on investing start to shift into unloved areas or even move away from market winners, they may know something useful or have an insight that you don't.
Recent history has demonstrated the edge specialists can possess. For example, discretionary fund managers who are multi-asset investment specialists tended to get out of open-ended property funds before the mass gatings of summer 2016, having noticed earlier warning signs that may have eluded investors with fewer resources and time. The majority of discretionary fund managers had also abandoned LF Woodford Equity Income Fund (GB00BLRZQ620) some time before it was suspended from trading in June 2019.
So assessing the current mindset of professional investors can offer clues, but remember that while this can feed into your ideas and provide an indication of broader sentiment, these investors can also make mistakes.