Purpose of attenuation before op-amp input
I cant find the actual reason for this topology

I can, here's an example:



This is an LM386 audio amplifier with an attenuator at its input. The attenuator is the volume control.

Since we have no idea what the input voltage from the source is this allows us to set the volume control such that the output level is where we want it to be. Within certain limits of course. Also the input voltage cannot be too small or too large.

If the volume control was not present at the input we would not be able to prevent the amplifier from saturating (clipping) when the input level is too high. 

Also, since this is a power amplifier, attenuating at the output is impractical as the power level is much higher (compared to the input), attenuating the signal at the output would mean severe power loss. You first make a lot of power and then waste most of it for a low volume. So efficiency is also a good reason to do the signal attenuation before amplification.