The Combination Bank holds the 100 Combinations available to the instrument at a time. This means that it is capable of playing more that one sound at a time. For example, it can play a bass sound and a guitar sound simultaneously. The basic purpose of a Combination is to define which sounds the instrument can play at a particular time. It also contains other controlling parameters.
Instrument memory is usually quite limited.
If you are making the most of your music then you will probably be customizing patches for each of your songs. The problem is the instrument's limited memory. It really doesn't take long to use up 100 memory locations. The answer is a Librarian. The Librarian can store the current settings from your instrument.