Most musical instruments need to be tuned, but the guitar--particularly the electric guitar--requires tuning surprisingly frequently. While many guitarists don't mind tuning the guitar once and then not worrying about it for casual playing around the house, virtually all tune their instruments before playing to an audience, or especially, before recording.
Until the late 1960s, that meant using either an A440 tuning fork, or a set of metal "pitch pipes" to tune from. But eventually, the Peterson company, who got their start after WWII building portable keyboards, helped to pioneer the concept of the "strobe" tuner in the late 1960s. Initially, this was a bulky, expensive product, so eventually, Peterson created a portable "virtual" strobe tuner whose current versions retail for around $200. In their review of their immediate predecessor, Peterson's now classic VS-1 tuner, England's Sound On Sound magazine explains the difference between real and "virtual" strobe tuners:
Real strobe tuners use optical techniques with rotating discs to generate their unique pattern of moving bars, which approach and then achieve stillness when tuning is achieved. The downside is that, being high-precision electromechanical devices, true strobes, such as the renowned Peterson and Conn models of the '70s and beyond, are bulkier and far more expensive than the typical electronic tuner used by almost every guitar player today. The reason that serious users maintain that nothing else will do, however, is that strobe tuners are more accurate. Significantly more accurate in fact; a typical pedal-type or hand-held guitar tuner will achieve something around a ±2 cents (hundredths of a semitone) accuracy, whereas a strobe (even a 'virtual strobe') will achieve a tenth of a cent, equivalent to one thousandth of a semitone.
Does anybody need to be that in-tune? Well it certainly helps, especially when you consider that all guitars are inherently slightly out of tune by a few cents on some chords at some positions. If the error in your tuner happens to coincide with the guitar's own deviation on any one note, you are certainly going to hear it. And if you do your own guitar setups, plus or minus even a couple of cents is really pushing your luck for bridge saddle positioning. The other thing about strobes is that they track movements in pitch effectively in real-time, so fine-tuning becomes a lot easier, as you can just ease up to a note, watching the read-out slow to a standstill. Overall, tuning is certainly faster and less frustrating.
Once strobe tuners became popular with touring guitarists and their roadies in the 1970s, numerous other firms began selling small, easily affordable electronic tuners, but they lacked the accuracy of their strobing big brothers. A strobe tuner (including virtual strobes) can be used to intonate a guitar, a procedure than helps insure that each string is as close as possible to being in tune at all points on the neck, not just in its first position. But most cheaper tuners aren't accurate enough for this procedure.