Do you find some mathematical activities useless?

[Dan]

Complex numbers is an example of a branch of mathematics initially considered "useless" in terms of practical application.

The Romans valued practicality and considered all mathematical abstraction useless. Commenting on the death of Archimedes at the hands of a Roman soldier as he drew geometrical figures in the dust, English mathematician Alfred North Whitehead wrote "No Roman lost his life because he was absorbed in the contemplation of a mathematical diagram".