Here's an excerpt from the article that I recently co-authored with Jerry Price another Vimist (just coined that term) for .net magazine.
There are many different fieldtypes, but one commonly used in a variety of ways is the text input fieldtype. For example, imagine inputting a 5x2 table of information. Through the use of the text fieldtype, 10 text fields can be added to the publish view of your entry. However, for much larger data sets, you can only wince at how cumbersome and tedious the task would be: 200 text fields running down the publish page. Not only would this overwhelm most users, it would also limit how much information they could input. Far from ideal.
Enter custom fieldtypes. By creating a fieldtype enabling the user to add, delete, and/or sort rows as well as input their data, the task would be much less of a burden. Not only a much more direct and streamlined process and experience for the user, this also facilitates customising the amount of information that can be displayed with a much cleaner interface.
You can find the rest of the article in the March edition of .net magazine.