To expand on @Septian Primadewa's comment
The winning pair is supposed to be able to claim the Holy Grail and utilize it to grant a wish for each of them. Its true purpose is to actually utilize the seven Servants returning to the root as a way to form a hole directly to it.
Source - Holy Grail War *emphasis mine
However the catch is in the last sentence of that quote. Most serious Magi are like Tokiomi Tohsaka and don't see the Holy Grail as a Wish Granter but a way to the Root as such they will keep one Command Spell until the end and use it on their Servant and force them to die as was Tokiomi's plan with Gilgamesh and why he plotted so much to regain his lost Command Spell
The Einzbern's would follow suit using all 7 Servants as they don't care for the Wish, they want the Third Magic, Heaven's Feel. much to the same as Zouken Makiri/Matou who sought the same magic because the materialization of the soul would allow him to continue to live but stop the rotting of his Soul.
For some Servants being summoned to the War is their wish, such as Diarmuid Ua Duibhne who wanted to faithfully serve a Lord which he was able to through Kayneth El-Melloi Archibald. Likewise with Masters as with Waver Velvet in a way, he didn't necessarily have a wish but winning the Grail War would show his worth as a Magus (seeing the Holy Grail War as the ultimate test of Magi)
Of course there hasn't been an incident where a Wish has been granted as all previous wars did not end with a victor. So there is no actual proof in universe that they are granted Wishes from the grail so the wish could be just to lure other Magi to the War and summon Servants so they can all be sacrificed for those seeking the Root or the Third Magic.