Grafing...Why?

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Makes a big difference in terms of soil diseases. That's the biggest benefit for the home gardener. For the pro, the advantages are more, like long season vigor, in the end they save money by keeping the same plant longer.
For cucumbers the advantages are insane, a neighbor bought some plants and had english cucumbers producing at the beginning of june, outside, after the coldest may in history, my (also hybrid) plants were still at the second leaf stage frozen in time.

For eggplant, again, in climate that is borderline adequate, will make a huge difference.

Look at the leaf of this grafted pepper, normally it was 5-6 cm long, I've grown it two years.

Look at the jumbo fruit of an asian eggplant grafted on just a normal tomato, not the super vigorous interspecific hybrids.