This is important because it allows you to combine two different classes of fonts: 1) good-looking fonts, that don't have broad glyph coverage, 2) less attractive fonts that have much broader glyph coverage.
It's just a fact of life that making a font look good requires a lot of author time. Making a font that has broad unicode coverage also takes a lot of author time. It's almost impossible to have both.
Being able to have fallback font functionality allows you to have the best of both worlds.