answers req :
In a brainstorming session, i could tell you the following:
For the first question, open loop systems could be instable too, for instance, suppose that you have a transfer function in your system that has poles on the right side of the complex plane. It will be instable, unless a proper feedback is used.
For the second question, in instrumentantio, for instance, you have to deal with noise, and if your system works on low frequency, there is a kind of nose called 1/f noise that becomes insignificantly at higher frequencies, so it is better to modulate your signal, to escape from this zone of noise. Besides that if you study an important equipament, known as lock-in, you will see that is syncronous detection is something very important.
I hope i could help you a little
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