This solves two problems. The first is a quality issue. By using one base glyph for many other glyphs, the hinting of this base glyph only has to be done once. This also makes all glyphs that use the same base glyph display the same all the time. Another benefit of composites is as a space saving device.
Outlines take up most of the font file's size.
Using composites can save many bytes in the final hinted TrueType font. Composite glyphs are defined differently than regular glyphs. Instead of outline data there is a description of the composite information. In TrueType editors high level commands are used to set this composite description. The Boolean is normally set so that the original glyph's bitmap shape is preserved.