A pilot test or pilot study is used to conduct preliminary analysis before going ahead with a full-blown research study or experiment. It is essentially a test run or rehearsal of the main experiment or study.
Small experiments can help test a hypothesis without costing too much, losing a lot of time and can be adapted to perfect the larger experiment. It can be used to help check that equipment works in the environment it needs to, that equipment used is the correct items and the statistics used are correct. They can help train researchers, allowing for mistakes and learning new technology and equipment.