I think if one ask young Thais,
Why do millions of people went out in the evening to put down small candle-lit and incensed sticks-lit flower floats along rivers or canals ?, I guess new generation people might answer that the festival 's just for fun. I think if they did answer like that they would probably be right. For hundreds of years, that how young men had chance to meet young women at a rare night festival like this. Some might say otherwise that it is chance in a year to pay tribute to the river angel (Phra Mae Kongka) for providing life line to peple's living. But I would say that explaination was based on Hinduism belief, not Buddhism's. I 'll put down Buddhists' view point down here, which hopefully would be more useful to Thai Buddhists.
Firstly for Buddhists, putting the float along the river 's mainly for worshipping the foot print of Lord Buddha who stamped his footprint at a place called Numtanatee (perhaps a river bank or an oceanside somewhere) almost 2.6 millennia ago.
Secondly, it 's to worship buddha's relics that had been floated at Ganges river in Varanasi by an Indian King after he discovered the relics in an old stupa near Varanasi.
Thirdly, the 15th waxing moon night almost 2.6 millennia ago was the day a prominent Arahant and Buddha's right hand premier monk, Sariputta (Thais called Phra Saribut), went to Nirvana (died). So this is also a remembrance for him.
Those are the reason why I and people in our group put down floats at the Ganges river at Varanasi 4 days earlier, while we were still in India.