toxic rain vs. caustic arrow

Hello again...
I would like to know, what the advantages/disadvantages are if one compares tr with ca.
And especially, why those high end builds I saw on ninja tend to use them both? Can anyone explain in detail what ca does better than tr and vice versa?

TR has high single target dps when built up but clear on it kind of sucks until you do ridiculous damage. CA is much better at clearing so some people keep both.

Strange, because I heard the exact opposite: Tr for crowd control, ca for tough bosses... Thanks for clearing that up.

The problem with TR is that to ramp damage, you basically need to stand still. Whereas CA especially with Arrow Nova, you fire one and it clears everything.