''In quantum field theory, the Casimir effect is a physical force acting on the macroscopic boundaries of a confined space which arises from the quantum fluctuations of the field. It is named after the Dutch physicist Hendrik Casimir, who predicted the effect for electromagnetic systems in 1948. ''

 

 

Title says "What is the Casimir effect used for ?". Possible answer: repulsive Casimir forces may have applications in nano-devices:

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In recent years, people have devoted substantial efforts to realizing repulsive Casimir forces, especially with a view toward applications to nano-devices and colloids, which can contain nearby parts that one wants to keep separate.

(Bold by me. Source: https://arxiv.org/pdf/1805.07994.pdf)

 

Paper on repulsive Casimir forces: https://arxiv.org/abs/1202.6415