There are a few main reasons.
Sound pressure spans so large range that it makes sense to use a logarithmic unit to begin with, like for many other things like Richter magnitude scale for earthquakes, f-stops in photography, pH for acidity and the pitch intervals like semitones and octaves for sound.
Also, why decibels? Well, 1 bel is inconveniently large, and 1 centibel would be inconveniently small, so for many things expressed in 1 or 2 digits, decibels fit just conveniently in between. And why notes are expressed in cents where 100 cents is one semitone and 1200 cents is one octave.
And the decibel isn't an unit, it is simply a ratio between two things, whatever they are. You just conveniently used it as amplifier gain example. The gain of an ampmifier is the ratio between output and input amplitudes, if in volts, the volts cancel out, and you get simply a gain factor, like 20, and can convert it to decibels, megabels, microbels or whatever measure you like.
So having a sound measurement and expressing it as decibels requires that there is a reference value to compare it with. It could be any reference. It could be pressure of some known sound level, or something like maximum possible sound level in air on planet Earth which has atmospheric pressure of about 1000mbar. In order to avoid negative numbers, and have an understandable reference, also the smallest sound pressure most people can hear could be used, and if done so, most of everyday life sounds are above 0dB and maybe below 120dB. Even the sound of a volcano which cannot exceed the 1 Atm pressure without clipping fits nicely to around 200dB. And smaller sounds you can hear can be expressed with negative numbers, and it simply tells you how much you need to amplify something to get to 0dB so you can barely hear it.
I also used the dB very liberally in my above context and like I said, the dB always needs a unit where you reference it to - hence it would be more correct to say that a lawnmower measures 80 dB SPL, as saying just 80 dB makes you think it measures 80 dB compared to what, grams in weight or milliliters in volume.