There are two kinds of guitar sample libraries: those which have individually sampled strings and those which do not. Guitar sample libraries without this feature are missing a whole dimension of the instrument. This is an important factor to consider when comparing different guitar sample libraries, or creating a sample library of your own.
Due to the nature of a guitar, there can be up to five of the same note found on different strings and frets. Sometimes it's a tonal choice, where on the neck to play a passage. Other times the strings and frets used are chosen due to how close they are to the fretting hand. In any case, this is why guitar sample libraries without individually sampled strings sound "flat".
Here is an audio example of how the same note played on different strings has a very different tone:
In the example (which uses CoreGuitar Strawberry), an E3 is first played on the high E string, then on the B, G, D, and finally the A string. As the note is played on lower strings, you can hear the tone progressively get darker and more "hollow-sounding". It's the same pitch, but played with a shorter (with proportionately less tension) amount of string.
This factor is what gives guitar its characteristic dynamic tone. That's why a guitar playing lead (generally played higher up on the fretboard) sounds so different than chordal playing (played lower on the fretboard). It's also a big contribution to making a sampled guitar sound like a real guitarist playing.
So far, I've only mentioned the tonal factor, but there's another reason besides tone that separate strings need to be considered. In terms of performance, each string is monophonic, while the guitar in whole is polyphonic. Building this into the guitar sample library is essentially for having it behave (and thus sound) as a real guitar does. It also allows you to play legato lead lines as well as rhythm guitar all within a single instance of the sample library. By calculating which strings and frets to use, other details such as picking style, string resonance, and legato threshold/range can be simulated. If you trigger your sample libraries using a MIDI guitar, you'll also really appreciate this.
Having individually sampled strings is an essential feature for guitar sample libraries, and a big step forward in realistically reproducing the sound of a guitar--it's the difference between the flatness of a 2D image and the depth of a 3D one.