How does insect pollination increase plant diversity?

EX:  We have Buzz the bee.  He's sent out by Queen Hun to collect pollen from the nearby flowers.  Buzz is a hard working worker bee, so he soon helps himself to the pollen in flower A.  But he's a bit messy; so some of that pollen gets onto his hind legs rather than into his stomach to take back to the hive.

But Buzz is not filled up yet; so he flits off to flower B, which is not the same kind of flower as A.  While he's sucking in the pollen from B, some of that pollen he got on his hind legs from A rubs off onto the stamen of flower B.  And that A pollen is absorbed into the B stamen and, lo, the next crop of flower B seeds have some genetic material from flower A.

Next springs flower B are no longer pure flower B.  Some of those flowers are now B with a bit of flower A in them as well.  They are hybrids of A and B.  Now we have A, B, and A and B; the flowers are now more diverse than they were the previous summer.

That's how.