They provide access to a range of asset classes and investment strategies that non-institutional investors may not ordinarily have access to, for example international equities, fixed income securities and currency markets. By investing in an ETF you can achieve a level of diversification that would normally be too difficult to realise investing in each asset individually. ETFs also have no minimum investment requirements which makes them more accessible.
ETFs usually have lower fees as passive investing strategies rely less on the skill and experience of the managers than active investing strategies. This makes them a cost effective way to diversify your portfolio and gain access to markets that traditionally tend to be more difficult to invest in.