There are several reasons that footage might need repair. Archival sources (especially those that are tape-based) can wear with time or equipment failure. Rushed crews or poor preproduction can also impact the quality of a shot through inadequate lighting. Environmental conditions, such as a sunset or inconsistent cloud cover, can wreak havoc with footage as well.
Although the reasons may vary, you'll find a useful collection of tools in After Effects. The footage we'll show you is bad (in this case purposely so).