Hi Mikhail

> Why should I use compressor Will it work bad without it

its quite simple, and the easiest way is to explaine with an VCA: Lets
say your guitar signal has a natural loudness shape with a decay phase
of 4 seconds. After 2 seconds the loudness will have become half.
This loundness shape is converted to the electrical envelope. The
envelope controls a module like an VCA, which will add the same loudness
shape additionally. The reuslting signal will now have after 2 seconds
only the quarter loudness: half loudness from the natural shape and this
reduced to half again by the VCA.

Keep in mind: most functions in a synth are based on a subtractive
synthesis, which means: there is a constant signal, from which I
subtract characteristics like loudness or brightness. The constant
signal is usually an oscillator. But in your case you have a guitar
string which does not sound constant. With the compressor we try to make
the strings sound as constant as possible.

Florian