I mean why does the phase angle varies on the transmission line even if we don't tap power in between those points...

The phase angle is generated at the three-phase generator. It is a function of voltage and not current.

When phase angle is a function of a voltage how does it varies on transmission line...
Y it is not function of curent...

Phase angle displacement on a transmission line changes due to two factors. One is the characterisitc of the load supplied by the transmission line. The second is due to the "RLC" characteristics of the line itself which occurs independent of the load placed on the line. Since the line is usually very long the effects of inductance and capacitance causes a phase shift to the energizing currect of the line. Since the line has a capacitance to ground, capacitance line to line along with the inductance of each wire and mutual inductance between conductors, the current will be displaced from zero phase angle to some value dependednt on the line configuration and length.

It is interesting to note that for say a 200 mile long line with no load on it, the line end voltage will be much higher than the voltage on the supply end of the line. This is due to the capacitance and in phase current with the inducatnce I spoke of above is often referred to as the Ferranti Effect

Hope this helps.