WHY PRAY AT THE GRAVES OF THE TZADIKIM?
The Talmud tells us that when there is a family member who is taken ill, that one should go to a wise man to ask him for a brachah. This rasises your very question! Aren't you in touch with Hashem? Why go to a human? do you need an intermediary? Isn't that idol worship?
The answer is that all of the Jewish People are connected to each other, and from that angle you and the wise man are one person. His merits and yours are bonded if you let yourself feel the bond, and the way that happens is by actually dong something that demonstrates your grasp of the fact that his merits and yours CAN BE bonded.
Maharal explains that this is even more so in the case of a tzadik who is no longer in this world, where the bond between you and him is completely spiritual. The more you identify with his deeds, his piety, and his Torah, the more your deeds, piety and Torah and his become one. One of the most dramatic stories in the Torah demonstrate this.
When Moshe sent the spies to report on Eretz YIsrael, he chose genuine tzadikim. Nonetheless, when they saw the power of the people, and the nature of the land, they felt that conquering it would be impossible by natural means, and that there is no reason to think that we could or should be pend on miracles. This was of course part of Hashem's plan-He wanted them to acknowledge that only He could help them make the land their own. Tow of the spies were able to resist the words of the o ther 10/ One was Yehoshua. Moshe had changed his name from Hoshea to Yehoshua before he left on his spy mission, and no doubt that was one of the reasons that he resisted the temptation to be lie the other spys. Kalev, the second spy who stayed faithful had no such blessing or name change. What kept him on track?
He broke away from the other spies (which meant being alone in a strange land with no one to cover for him or help him defend himself if he was captured). He went to Chevron to pray at the tomb of the Patriarchs and Matriarchs. The Talmud questions whether he did the right thing, since separating from the group was a real danger, and endangering your life is forbidden. The conclusion reached in the Talmud, is IT WAS SAFER FORE YEHOSHUA TO GO TO THE TOMB OF THE PATRIARCHS TO EVOKE THEIR MERIT, THAN IT WOULD HAVE BEEN TO STAY IN THE GROUP. This means the value of the merit of his going to pray to their tomb is actually what saved him.
Going "from grave to grave" with explanations of what these tzadikim acheived in their lifetimes, can be a source of deepening your connection to them, and more significantly, bond their merits with yours.