Sometimes just reading the words of a prayer may not be enough. Hashem wants us to change through our prayers and segulos help us with it. Whichever way it happens but once we change then the decrees against us may change. When you go to a kever of a tzadik to pray somehow your kavonos may be a bit stronger. Because by doing something extra (something you dont usually do) you feel like your prayers will be more likely heard and answered so you concentrate on them better.
So i think segulos are needed for some of us to fool ourselves into praying with more attention. And once we do that enough times im sure we will change and our mazal will change with us.
(I think people can make up their own segulos as long as it leads you to pray better and to grow spiritually then it should work. However, it is obviously better to follow our tradition and only use the segulos that were passed over to us.)