The Quaternary alluvium (that means sand to ordinary folk) around Botany Bay supported nice green swamps where poverty-stricken plants struggled for an existence, but the gentlemen botanists saw none of this, not when there was so much green around them. They gathered specimens, and Banks schemed to have this demi-Eden turned into a British settlement. It mattered little to him that the plants were growing on some of the worst soils to be found on this planet -- to him, it was a paradise of plants, and Cook was prevailed upon to change the name "Stingray Harbour" to "Botany Bay".
So when Banks rose to a position of power in England, he used his influence to send a settling fleet to Botany Bay, ready to create a power base for Britain in the South Sea. From here, Britain's navy would be able to refit, and perhaps ship yards could build vessels to fight against the French who were, after all, The Enemy.
Oddly enough, a French expedition put into Botany Bay just a few days after the First Fleet arrived, and fools often say that Australia nearly became French, on account of that arrival. This is wrong, for the French knew the British were settling in Botany Bay, the English knew the French would be popping in for a chinwag and a quick bite to eat, and the meeting was all arranged, long before either group sailed from Europe. In those days, scientists were above ordinary warfare, and the British government had instructed Arthur Philip to extend all courtesies to the French visitors.
What is important is that Botany Bay, for all that it is a fine harbour, was no place for a settlement -- no water, poor soil, poor anchorages, shallow shores -- and so Arthur Philip took off for the next major gap in the shore, and so discovered Sydney Harbour. Botany Bay was left alone, low swampy wasteland, where Sydney's airport would later be built -- you can see one runway jutting out into the bay -- and settlement happened further north, at Sydney. And so Sydney was formed, although to two generations of British criminal under-class, this land was "Botany Bay".