Since XRender introduced transparency, it was now possible to treat partial pixels along the edges of geometric objects as if they were translucent and thus approximate anti-aliasing.
Also, XRender provided a way for applications to cache glyph images within the X server (in a GlyphSet) and then to rasterize a sequence of cached glyphs. Each Glyph in a GlyphSet is essentially a Picture with additional geometric positioning and glyph advance information. XRender also allowed for the incremental rasterization of glyphs, thus eliminating the inefficiencies of rasterizing every glyph in a CJK font, for example.
When XRender was first developed, there was concern about the increase in network traffic caused by the need for clients to send rasterized glyph images to the server for caching. However, tests proved that the glyph image traffic was more than offset by the fact that the X server no longer needed to send font names and glyph metrics to the client.
Note that Xrender does not itself have font support: applications are responsible for locating and rasterizing glyphs and obtaining the geometric information associated with glyphs.
This led Keith Packard and others to develop Xft ...