Let me give an example. Most students would, at some point in their religious life, do a retreat of Tara. Because Tara attainment comes quickest of all, they certainly would have gained some attainment from that. When they come to practice another deity, such as Kurukulle, Norgyun or Saraswati, they can connect through Tara. Tara gives attainment very quickly and also, she is very tolerant of mistakes or omissions in the practice, so that one gains those benefits when doing a combined form.
Combined forms are common in Tibetan buddhism as transitions from one practice to another.
Also, by doing one combined form, one is saved having to do many different practices. For example, rather than do the practices of Hayagriva and Guru Drakpo and Garuda and Vajrakilaya, one can do Guru Drak-Pur and combine them all. Or, for another example, it can be quite difficult to enter into the practice of the Vajra Heruka.