Already this might have discussed. The growth of bitcoin as well other digital currencies keeps progressing with time. By the time common people too have begun to use cryptocurrencies on their daily life. Compared to the fiat usage, common people using cryptocurrencies were low. When the usage increases what will be the preference of common people. Whether it gets used as investment, gambling asset, trading asset, alternate payment, darkweb usage source.
considering the crypto space currently, Bitcoin is used in all aspect of the things you have mentioned above. People are either investing into it or trading, gambling and using it as an alternative payment option. There are no particular aspect which can be considered as to have the most increased usage.
Everyone may have unique purpose on using bitcoin but we can tell which of the choices OP given are bitcoin is mostly in demand.
Investment and trading does not increase an increase of usage because they are just using bitcoin to gain profit based on the price fluctuation, but when we say of usage, when people likes to gamble and the crypot space offers a lot of gambling sites that are competitive, that way a demand is created.