Why is tert-butoxide often used in elimination reactions when it is not necessary?
Several reasons:


t-BuOK is especially known as a strong base, and a poor nucleophile. Its large, bulky structure causes it to perform exceptionally poorly in substitution, literally eliminating any side reactions when the desired product is the elimination product.
It is easily available, like Raphaël insists.
It's basic strength depends on the medium. It is very strong in DMSO, where the solvent complexates well with $\ce{K+}$ ions. However it is relatively weaker in benzene. This flexibility gives it inevitable popularity.
Many condensation reactions (Stobe's, Darzen's) are seen to have greater yield when t-BuOK is used.