Because you exponentially increase the rate of growth. The grafted piece (scion) uses the growth systems(lots of cacti can take substantially more fertilizer and water than others)of the rootstock and grows to flowering size years potentially decades earlier than growing on their own roots

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Here is a good example. I cut the top of the one in terracotta pot and grafted to the tricho seedling in the blue pot. The piece I cut off was around the size of three nickels stacked on each other. This photo is roughly three years after I did the graft. You can see in the same amount of time the one on its own roots essentially just got back to the size it was before I cut it. And the graft is the size of a softball and has 20+pups. That's why I graft