So you can invest in any idea or theme that suits, big asset managers package up those ideas into one fund which is then broken up into units (the ETF) for you to get that exposure more simply. Essentially, that's why its called an ‘Exchange Traded Fund’, but is as accessible as a single stock. 

Through ETFs investors can gain exposure (with either long or short bias) to a variety of asset classes or individual sectors with the ease of transacting in a single security. ETFs can be made up various securities or assets, including stocks, bonds, commodities, or currencies and are typically designed to track the performance of that asset class or idea.