window functions - What is the relation between windowing and hopping in audio DSP

While using short-time Fourier transform (STFT), it is common to use window function, usually tapered.

• Will this not alter the original signal $x[n]$ and the STFT may no more be the true STFT of $x[n]$?


• Also, what is the concept for sliding the frames not by $100\%$ of its width but a fraction of that? i.e, why should frames hop in small steps (for $75\%$ overlap, frames hop by $25\%$). Is there a conceptual explanation with as little math as possible?