Short answer its just for fun, mostly you should just use them up as you get them.  Theres interest given on the balance you have hence the prize compounds in that small way.

However price in my guess is descending at present which gives the slight side benefit of about 6% appreciation between the prize yesterday and prize/price today.   However BTC has been going in circles recently anyway, you wont get billions this way its my personal game to guess.    I won the bigger prize but it was off a free wheel spin actually: