Can you flesh out and explain further the link between the physical act of meditation (focusing on the breath, etc.) and the spiritual benefit it provides? Is it that you turn off all the other senses and are just left with what can be defined as your true self/soul? How does meditation on a physical level help you come closer to merging your soul with the infinite?
God cannot be perceived or contacted easily when our minds are stirred up by constant thoughts — thoughts primarily generated by our egoic desires.
Thus it is necessary to find a way to quiet the restless mind, leaving the doors open for direct perception of God and our own God-Self or Divine Reality (the soul).
God is very "polite," and has given us free will. He/she will not impose him/herself over our desires to be separate or to indulge in constant ego driven desires and restless thoughts caused by them — or to "talk" to us over the "noise" of all that stuff.
We must, of our own volition, make a conscious decision to concentrate/meditate on and in silence, to achieve that a true calmness, wherein God’s "voice" may be heard and divine contact is possible.
The physical breath and the mind are directly connected to each other. As the breath goes, so go the thoughts and visa versa. Great masters of yoga saw this fact and understood that the best way to calm the mind is to use physical and scientific techniques to calm the breath.
In the process of learning to still the mind and breath and in the deep inner peace, which comes from that process, we begin to realize the true Self. But we must start with what is given to us (as physical beings, the breath, etc) to achieve that peace.
Yogananda also mentions, as you do also, the importance of "turning off the five sense telephones." So, to cease seeing, hearing, tasting, touching, or smelling is a great aid to deep meditation, for these are the "vehicles" which cause sensory stimulation, which leads more thoughts and continuing restlessness. In turning these off, what is left is an aspect of God which is Peace and even deeper than Peace, Untouchable Calmness and Ever-New Joy — all aspects of God!
So we use the physical to find the spiritual and eventually find there was no difference between the two, in the first place.