Chip designers realized that many of the advantages of 3D integration could be approximated by placing bare dies side by side on an interposer instead of stacking them vertically.
If the pitch is very fine and the interconnect very short, the assembly can be packaged as a single component with better size, weight, and power characteristics than a comparable 2D circuit board assembly.
"[4] Some benefits: Some sophisticated 2.5D assemblies even incorporate TSVs and 3D components.
[7][8][9][10][11] The success of 2.5D assembly has given rise to "chiplets" – small, functional circuit blocks designed to be combined in mix-and-match fashion on interposers.
Interposers can be larger than the reticle size which is the maximum area that can be projected by a photolithography scanner or stepper.