Filters could be seen as small plug-in programs that integrate mathematical or logical procedures. Some of them work on a per-frame basis while others have different temporal dependencies. Resizing and histogram adjustment are examples of the first category while de-interlacing and motion blur (which depend on more than one frame in time) belong to the second category. They are very useful because:
1. They can be combined in a chain fashion (the relative order can be important) applying various effects without producing more than one output.
2. They are easy to use although they provide essential functions.
3. They are modular so you can easily add or remove them.
4. Many of them are free. Check out http://neuron2.net/.