Number fields should be used whenever a field represents a numerical value. They only accept numerical values, and you can set a minimum and maximum value.
Use number fields for doing things like prices. Leave out the currency ( you can just hardcode that in your templates ). Then you've set the stage for doing things like properly sorting entries by price in your templates.
Eg. entries with number field named "price" could be sorted in ascending order like so:
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<pop:entries order="price ASC"> <p><pop:title /></p> <span>$<pop:price /></span> </pop:entries>