Basic idea:
if user have download package, then when upgrading this package instead of downloading full .deb file from server, download only changed parts.

Rationale: if I already downloaded XYZ-1.0, theoretically I can reconstruct XYZ-1.1 using XYZ-1.0 and diff-like file.

Pros:
- It can reduce traffic, that one need to waste for upgrading packages. In some countries traffic is still expensive, and speed is still low (