Why would anyone want a 1, 2, or 3 string guitar? Why get a roots instrument at all? Why not just get a cheap guitar from a music store of some kind and go from there?

Uniqueness of sound. No two instruments sound or play exactly the same. You can get sounds out of a 1, 2, or 3 string, roots style guitar that you will never get out of a conventional mass produced instrument.

Another reason is the shorter learning curve. You will be able to play actual songs on a reduced string roots instrument in an amount of time that would have you still trying to strum out scales and learn chord shapes on a conventional guitar.  You will easily learn to play power chord based rock riffs and rhythmic grooves on a roots instrument in an open tuning that would be difficult to learn on a conventional guitar in standard tuning.

The style and cool factor. There is nothing like playing a deep groove on something made my hand and probably looks like it shouldn't even work. Nothing has the feel and the soul of a handmade roots instrument.