According to Microsoft
Microsoft® Windows® Installer is an installation and configuration service that reduces the total cost of ownership.
The second part of that statement might be open to debate but the infrastructure does provide flexibility and some sophisticated capabilities for shared installs, JIT installation, remote upgrades etc.
The Windows installer can be accessed through COM automation. Below is an example using TWAPI that enumerates installed products and versions. The same can also be done using WMI as illustrated in TWAPI and WMI.