Indeed, on many softsynths curving an envelope stage is a parameter in the envelope itself. By using the modulation matrix, the virus offers some flexibility though:
Modulating, for example, filter decay rate by the filter envelope allows for snappier decay rates, but negative modulation also allows for more convex behaviour. So positive or negative modulation allows for both concave and convex curves.
On top of that, every source in the modulation matrix can be used to modulate the depth of another slot in the matrix. In the above example, one could use a tempo synced lfo which modulates wether the decay rate modulation is positive or negative, allowing for a tempo synced change from convex to concave decay slopes.
Also, lfo' s can modulate their own modulation depth, their own amplitude, their own speed, etc. etc.
By using the modulation matrix for recursive stuff an enormous amount of flexibility emerges since such an approach is more generic than using dedicated parameters.
lfo 1 is modulating the depth of lfo 2 which modulates speed of lfo 1 which modulates depth of lfo 3 which modulates both filter cutoff and lfo 2 and 3 speed.
On the virus the above insanity is possible. On most softsynths it is not.