Why do we still use vacuum tubes in modern electric guitar amplifier?
The vacuum tube, electron tube, valve of simply tube is a technology that was developed at the beginning of the 1900. Its primary purpose was to perform fundamental electronic functions such as signal amplification and current rectification. The invention of the transistor, a semi-conductor device,  in the 1940s presented a better, safer, smaller and more efficient alternative to the vacuum tube. The transistor also have the advantages of being more reliable that the vacuum tube. Despite its inconvenient, the tube remains favoured by many audiophiles and musicians. 
In electric guitar amplification, the tube produces pleasing harmonics when pushed into saturation. The tube generally offers a softer clipping characteristics compared to bipolar transistor when the amplification gain is pushed. It is also known to react extremely well to the guitar player dynamic ranging from clean sparkly sound, when barely touching the guitar strings, to crunch and distortion when entering saturation.