String theory is needed because ordinary particles do not settle the discrepancy between the quantum mechanical frenzy and the general relativistic "smoothness"! In order to merge all of the forces of nature i.e. EM, Strong, Weak, and Gravity, you need particle-like entities stretched out so to speak i.e. a string. That's why strings are needed! However before 1995 there were originally five different string theories which were called Type I, Type IIA, Type IIB, heterotic SO(32), and heterotic E8XE8. Ed Witten during a conference in 1995 presented his elegant unification of all five string theories i.e. M-theory! Witten showed that the five string theories are actually part of a larger framework. So a string is a one-brane. A two-brane is a sheet. A three-brane is a three dimensional sheet which you cannot visualize. And according to theoretical physicists, our universe is a three-brane because of the three dimensional space we inhabit. The highest dimensional brane in the M-theory multiverse is a 10-brane. The 11th dimension is time. Time can be followed through the other side of our universe's singularity!
Even though there's no evidence for M-theory, I have a braggadocious attitude that the Multi-verse is real because it would explain our universe's complexity and "perfect" cosmological expansion rate without the need for a "god". In the same way the Earth is "fine-tuned" within our solar system.
Fascination with M-theory!