.Value necesary?

I have found that in some situations I can leave off the .value part of a form object and the statement will still refer to the value of the object.

For instance Option Buttons can be tested with just their name, and no .value.

Are there only certain objects that this works for, or is .value superfluous?

most objects have a default property.

for option buttons, checkboxes textboxes it's .Value
for labels it's .Caption

If you use the object browser the default properties have a green tick/blob on the icon.

Although it is possible to code using the default property, that is not specifying it, it is better practice to use it.
It will make your codes intent much clearer when revisting your code.