why chemicals like KI solution, sodium bicarbonate , etc. are used in the experiment estimation of Cu in CuSO4 using Na2S2O3 solution?

Copper ions are reduced by the iodide ions to copper(I). The best way to look at this is via the half equations:

Cu2+ + 1e --> Cu+ 
2I- --> I2 + 2e

to balance these you double the first equation (to equalise the electrons) and add them together.

2Cu2+ + 2e --> 2Cu+ 
2I- --> I2 + 2e
------------------------------------------------------------- add
2Cu2+ + 2I- --> 2Cu+ + I2

Now add in the spectator ions:

2CuSO4 + 2KI --> Cu2SO4 + I2 + K2SO4

So the white precipitate is copper(I) sulphate and the brownish coloration is iodine solution...