If the electric field hits the receiving antenna (in form of pressure or voltage) to agitate the electrons in the receiving antenna, then what is the purpose of magnetic field then? What it serves for?
Electromagnetic waves are independent of the source, once generated, because varying electric fields and magnetic fields can move through the vacuum without need of reinforcement by a source ( as happens in an electric circuit). They become independent. In the classical mathematical description of light
Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D animation shows a plane linearly polarized wave propagating from left to right. Note that the electric and magnetic fields in such a wave are in-phase with each other, reaching minima and maxima together
So the magnetic field is necessary for the existence of the self propagating wave.
@annav 's answer illustrates very nicely that, for the wave to propagate from the emitter to the receiver, you need both the electric and magnetic field.
To simplify matter, suppose the wave propagates in air. Then the ratio of the magnitudes of the electric to magnetic field is constant (and about 377 if you care to know). The electric and magnetic field both decrease in amplitude at the same rate, which by geometry is usually $1/r^2$ with $r$ the distance from the emitter to the receiver, i.e. if a first receiver is twice as far a second receiver, the first will detect an amplitude of 1/4 compared to the second.
The propagation is such that you cannot propagate an electromagnetic wave by propagating the electric or magnetic field alone: both are required. You need the varying $\vec E$ to produce a varying $\vec B$ which in turn produces a varying $\vec E$ in perpertual induction one to the other.
Once the wave reaches the receiving antenna, only one or the other field will usually "couple" to the receiving antenna. The details depend on the antenna but the simplest cases are the short line antenna (or Hertzian dipole) or the small loop antenna. In the case of the latter, it is the magnetic field that drives the antenna, while in the former it is the electric field.
Please be aware that the above two cases are just the simplest and that antenna design is quite a specialize field of electrical engineering, with some antenna design cleverly optimized for specific applications.
Thus, neither field is really redundant since
- they are both required for propagation, and
- which one will "activate" the antenna actually depends on the antenna, and there are probably models where both "activate" the antenna.