ETF providers have increasingly aimed to provide funds for investors looking to invest in a narrow market segment. The ETF universe is flush with funds that focus on a single market sector, industry, or geographic region. Say you favor indexing and want to own a specific corner of the market such as biotech. There may not be many index mutual funds that track those sectors--but there are ETFs that do. Also, there are many more ETFs than funds that track single foreign countries. Exchange-traded funds offer investors a way to invest in a corner of the market without having to load up on just one or two individual stocks (plus, it's more cost-efficient in terms of brokerage commissions).
However, it's also worth noting that narrowly focused funds--whether ETFs or conventional offerings--can be too hot to handle for many investors. That's because investors are often inclined to buy and sell narrowly focused funds at inopportune times.