The guitar is a diverse instrument. When I meet a guitar player my first question is what kind of style do they play? Singer songwriter? Jazz? Rock? Classical? Flamenco? Even within rock you can specialize in various sub-genres: shred, 80s, classic rock, surf rock, punk, speed metal, etc. Each style demands a unique musical approach and technical proficiencies on the instrument.
The use of a guitar pick is one example of a technical difference in playing certain styles. In classical and flamenco guitar a guitar pick is not used. For the 80s rock style of electric guitar that I play a guitar pick is mandatory.
Guitar picks come in all shapes and sizes. The primary difference among them is the thickness, measured in millimeters. A very thick "heavy" guitar pick is 3.0 mm. A flimsy "light" guitar pick is 0.7 mm. You can find a pick any size in between: 2.0, 1.7, 1.0, etc. The weight of the pick impacts not only the tone produced by the guitar, but also its playability.
For tone, you have to consider the type of music you are playing. Staying with the 80s electric guitar theme, if you're playing the rhythm guitar part of "Crazy Train", a light pick will produce brighter tones which are desired when playing the chords in the song. A heavy pick will produce duller tones that will make the chords sound bland. A light pick will glide across the strings with less precision which is exactly what you want when playing rhythm guitar. A heavy pick will be more abrasive when strumming chords, and hence will result in duller tones.
Things are flipped when you reach the guitar solo in "Crazy Train". The flimsiness of a light pick will no longer be advantageous as the lack of precision becomes a negative. Playing a lot of notes fast will become a challenge because the light pick lacks the precision the heavy pick has. Making a change between a 0.7 mm pick to a 2.0 mm pick could make a tremendous difference in not just guitar tone, but the playability of the solo. With a 0.7 mm pick I would likely miss notes and the solo would sound messy. Yet the precision of the 2. 0 mm pick would lower the risk of missing notes and likely make the notes in the solo cleaner and brighter.
A difference in millimeters can make a huge difference on the tone and playability of your guitar.