as i understand, they provides storage of data of many formats, and internet data streaming. they natively support java script, web gl, c++. it's convenient for website developers to put 3D images into their sites. and games can make use of it too.
i think heightmap data is one of the data types. 3d models like buildings are stored as varies formats.
www.geo-fs.com is an example of the usage of Cesium.
but java is not natively supported.