Is it true you shouldn't rely on placeholder alone for form input fields?
Because putting the same text as the title seems reduntant:
Is there a reason why a title is necessary for simple/self explanatory form fields?
To answer your question "Is there a reason why a title is necessary for simple/self explanatory form fields?"
No it is not necessary for "simple/self explanatory" form field. If you have a simple form you can easily manage with placeholder. For example, Name, email and age.
However the answer is YES when there are
1. More number of fields (long forms) - User needs to keep track of all the filled input box.
2. Similar types of field (multiple text fields or numeric field) - If there are multiple text field or numeric field then user will not be able to distinguish between them
3. If values are getting auto-populated - If form auto-populates values then user would not understand what the field is suppose to be for.
Displaying title on fields is always a better way ensure proper readability. At times the titles are sacrificed to achieve minimalist designs. However the context of the field should not be lost in such scenario. In case of important details like input box for entering credit card numbers; it is always better to display the title (it gives better readability).
A better approach to avoid redundancy and have both (title and placeholder) is to see how material design achieves it using floating labels for text field. check here: https://material.io/guidelines/components/text-fields.html#
The issue with using only the placeholder text instead of a proper label is that as soon as the field is filled out, the placeholder disappears and the user has to remember what goes in what fields.
On a simple form with only two or three inputs that's not too bad, but imagine a big form where you have dozens of input, and they all text in them : how do you know what goes in each input?
It's even worse if you have to prepopulate the fields (ex: in an edit form) because then the users don't even have a chance to see the placeholders once.
In your example, I wouldn't put the same text as the label in the placeholder. Either leave the placeholder blank, or you can also use it to provide extra information about what goes in that field (ex: password validation rules, input format examples, etc). Even that is not ideal, and some would argue that it would be better to include that besides the label or below the input.
Because putting the same text as the title seems redundant
A Placeholder is not necessary the same as the label of the input.
You can use to complement the information required.
| Write your real name |
| Use 10 digits without slashes or spaces |
Now regarding the question.
Imagine you are filling a form where you can registry your Real name and an Alias to be displayed
| Real Name |
| Displayed name (Alias) |
But you rely only on placeholders. Once you filled both can look.
| Rafael |
| Rafael |
On a second thought, I do not want to display my real name... And because both are now the same, I could update this form in a wrong way.
Another example is when the user wants to visually confirm all the fields are filled correctly. It is more useful to have a label so you can pair your information.
| 55555555 |
| 66666666 |
So. It is true.