By virtue of their electrical properties, tubes generate a special waveform when they're saturated, which is why tube engineering has tremendous tonal advantages over solid state or DSP solutions, particularly for crunch and lead sounds. Tubes enter the saturation zone gradually or softly, which lends tube-driven tone its trademark yet totally unique character.
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They are used for accurate playing. It helps to make sure you play accuratly at whatever speed you want. Make sure you start off slow ;). I wouldnt say it is important when you first start playing guitar but it does come in handy when you do get a little better and especially when you want to shred lots. Without using a metronome you will probably just become really sloppy and miss lots of notes !