Why is deconvolution used in telescopes, CT scans and seismographs?
As long as the "distortion" is caused by a linear system, its effect can be modeled by a convolution operation with some "filter". The filter is different for different applications and perhaps even different scenarios and is usually measured / estimated before the deconvolution operation.

The deconvolution itself attempts to recover the original signal given the measured output which is the convolution of a signal with a filter (essentially the signal was filtered and we use the result to try and recover the original signal before filtering).