Storage of value is an application, currency is an application. I think Bitcoin is one level of abstraction higher than an application. It is a protocol that can be used to run applications. How effective it is in any given application is based on properties of the protocol and its current state (i.e. price, volatility, mempool size, block reward rate, fees, transaction throughput etc.)

Then there is 2nd layer protocols to consider which would extend Bitcoins ability to be applied in an application invariant of some of these properties. For example the Lightning Node Network would allow bitcoin to be applied in the application field of common currency invariant of the protocols transaction throughput. Besides, currency and storage are just two possible applications and there are others that Bitcoin can be applied to like asset management (e.g. Placing land titles on the bitcoin blockchain).

If your question is what application is Bitcoin currently suited to, that is a bit more of a complicated question as we need to look at the environment which Bitcoin is operating in. External factors like fiat currency hyperinflation causes people to use Bitcoin for purposes that it may not technically be suited to at this time (like storage of value). Whilst its technically not suited for storage of value purposes yet, people see fit to hold a portion of their value in Bitcoin and not in fiat or in any other asset for that matter.