If the term of butsudan simply means a miniature temple, it should not be necessary for any temple which has already its own main hall for Buddha, but an onaibutsu altar is additionally installed in many temples considering it serves for the spirits of ancestors.
小型の寺院という考え方であれば本堂があるので必要がないはずだが、先祖供養の観点から別途用意されることが多い。