Its original notion was "peer-to-peer electronic cash". As best I can tell, there's almost no significant legal use beyond speculation. [1] Even prominent Bitcoin advocates have admitted it isn't a good currency. [2] Illegal use is self-limiting; if Bitcoin becomes mainly known as the crime currency, it'll be truly screwed, as governments everywhere will further crack down on conversion and use, driving off investors, developers, business partners, etc.
I think it's especially obvious how much it's failed as e-cash when you compare it with M-Pesa, an African e-money system. [3] It started just a year before Bitcoin, but does something like 100x the transaction volume and is wildly popular in a number of countries.
[1] E.g., https://www.nytimes.com/2018/04/16/nyregion/new-york-today-l...
[2] E.g., https://avc.com/2017/08/store-of-value-vs-payment-system/