Over time, very few actively managed funds outperform index funds. Index funds cannot beat the benchmark because they ARE the benchmark. Similarly, they cannot fall below the benchmark. Actively managed funds can, and probably will, do both over the course of time. So if you prefer less fluctuation in your investments compared to actively managed funds, an index fund could be a good idea.
Index funds do fluctuate, but only as much as the market does.
This does not mean that you should invest only in index funds. Well-run actively managed funds can give your returns a boost, especially if you have a long time to go before you retire. But you still might want to consider keeping some core holdings in index funds. In almost all cases, index funds have outperformed managed funds over time. Index funds provide a degree of certainty in an investment portfolio, which to many investors is more important than the possibility of great performance but also more risk.