The methods that come with the standard ActionPack::Helpers::FormBuilder cover most cases of what you need to do but if you need to do something it doesn't have a method for, things can get a little ugly.
In my case I wanted to create a helper for autocompleted fields that all had the same attributes but different arguments (the urls).
So let's say you have an Unobtrusive JavaScript file that looks for an attribute called data-autocomplete, normally when you wanted to have the field autocompleted, you would pass in the attribute to the form helper text_field.
f.text_field :username, :'data-autocomplete' => users_path(:json)While there is nothing bad about doing this, you end with a bunch of :'data-autocomplete' scattered about in your views which makes it harder to change down the road if you were to switch your JS implementation.
A nicer way is to have your own form builder method called autocomplete_field which requires two arguments (method and url) that adds the :'data-autocomplete' option (which is now in one place and easily changed) for you. Best part is this can be done with only one helper file.