Why I-beam's are preferable over rectangular beam's?

You just take an example as rect section with A=17.6*10=176 mm2 and your I section too has same area of 176 mm2.

Calculate moment of inertia of rectangular section I = bd3/12 = 1466.66 mm4

For I section, Width of both flange = 20 mm, thickness of both flange = 4 mm, web length=16 mm, web thickness = 4mm. This gives you the same area A=176 mm2

Now Calculate moment of inertia of I section I =8938 mm4 (Do it from any online converter or by calculations)

Now compare both Moment of inertia, I section has approx six times better moment of inertia as compared to rectangular section. Put up this moment of inertia values in deflection and bending stress equations and try to compare both. This is because the material is put up in such a way to get maximum moment of inertia with minimum material and min weight. Finally this is the reason why I beams are preferable over rectangular beams

Once Put up this moment of inertia values in deflection and bending stress equations and try to compare both. you will get it in sec