Small cells are a critical piece in many network operators' plan. At the core, they provide network coverage to small areas. While tall and high powered macro cells keep the network signal strong across large distances, small cells are much more suitable for either densely populated areas like shopping malls, concert theatres, etc. or coverage enhancements in difficult terrains and/or rural areas.
Another goal of small cell technology is to improve the cellular experience for end users. It strengthens coverage and data transfer speeds where devices might otherwise compete for bandwidth. And small cells have been shown to extend handset battery life by reducing power draw, so devices can go longer between charges.