Precipitation of Ag+ and S2-

[blitz.km]

Why do we have to add S2- slowly and gradually for precipitation to take place??

[blitz.km]

I am sorry for a wrong question..
Actually, its as follows:

Why do we add S2- slowly to Ag+ while experimenting ??

My teacher has answered it saying that if S2- is added quickly, we won't be able to study the reaction.
Its added slowly, so that we can note at what conditions does the precipitation occur.