They’re the bare bones of harmony, the three elements combined to make what we call 'chords'.
They are seen and used in countless songs across vastly different genres: rock, pop, funk, folk, reggae, ska (even metal!), which makes them essential learning.
There are literally hundreds of ways to play any chord if you understand how chords are built and how you move them around the neck.
Not only will the study of triads revolutionise your rhythm playing, it will also ensure you hit the right notes every time in your lead solos.