A currency is mostly a measure of value. But as far as other currencies exist, it works also a commodity when it is traded for other currencies.
What is the most important factor that impacts the value of a currency ?
Most people would answer: the quantity of currency in circulation.
Wrong: an increase of the monetary base (the total currency available) isn't necessarily linked with an increase of prices, that is with a devaluation of the currency. And for sure this is not the main factor: a currency can be very rare, but still be worthless.
The main cause of the value of a currency is:
the quantity of people willing to accept it, and the eventual preference of the currency over other currencies, that is the quantity of products that can be bought by this currency.
The most a currency is a accepted, the most it is valuable.
And of course the most a currency is valuable, the most its value increases on the currency exchange market.
Accepting Steem, and preferring Steem over other currencies, can accellerate Steemit acceptance, and create a virtuous circle, so it is the best way to invest in Steem: accepting dollars and then converting dollars in Steem could look similar, but it is not, because, also if it increases the request of Steem in the currency exchange, it does not increase the quantity of products that can be bought by Steem.