This is an “eco roof”, probably by accident rather than design.
A “flat” roof should in fact have a slight gradient on it to encourage easy drainage of rainfall.
The amount of vegetation here suggests little or no gradient, the resulting moist conditions creating an ideal environment for you to prove your green credentials.
The problem longer term is that such growth and moisture retention will accelerate deterioration of the flat roof covering meaning you are left with a bill for repair or replacement, possibly sooner than you had anticipated.