When would you use a drop-down field?
If there are many more than 10 options, and the optional input either has to be submitted in a known and structured format for validation and analysis, or where users don't know their options upfront (hence, can't type it), a drop-down can be a completely warranted choice.
How many types of drop-down are there?
I also want to clarify that there are two main types of dropdowns: those used for navigation and those used in forms.