It's useful in the sense that once you get the idea of verbs changing with the grammatical person, you'll also know "Slaap jij?" "Slaapt hij?" "Slaapt zij?" "Slaapt het?" "Slaapt men?" "Slaapt u?" "Slapen wij?" "Slapen jullie?" and "Slapen zij?" Knowing eleven sentences for learning just one, looks like a fairly useful way of learning things.
Context, I assume.
Although there is a specific construction for the continuous aspect, built using a form of zijn+ aan het (bare infinitive verb)
In the case of this sentence, if you want to make it absolutely clear it's the continuous aspect, you'd say:
ik ben aan het slapen.
But the use of both of these cinstructions is not as clear-cut as in English.