Some forms are really long and detailed to the point that your surprised certain info is asked.
Some ask for a lot of personality details.  And other's are vague and short.

Personally I enjoy reading a well thought out form. If the form is done right it tells you about the player and the oc. And in some cases gives you a more accurate mental visual of them.

If the form is to vague... It leaves a opening. In fact I politely turned a rp down semi recently partly due to the form after being tagged to it.

Why, because the form was so vague for a fantasy rp, It could lead to a pandora's box of issues. In The roleplay literally left its self open to over powered ocs, God Mods, and mary sues. Though its aided by vague rules.

Why are they a issue? Cause they usually lead to fights, people quitting or rage quitting.  And eventually...
A dead roleplay. 

Though sometimes the person filling out the form is vague. You'll see is a popular answer. In fantasy rps its often put in powers or abilities. Though I've seen people use it in every slot but name.  Usually these are heavily rejected by smart admins.

Another good use for form is it tells you if they read the story to the rp, and the rules. If you ask for the password in the form,  then depending on where in the rules they read it, it should be in the form.

Characters backstory can also tell you if in anyway it includes a canon rp character and they didn't plan with you.  (This can be resolved by a connections page also)

Forms can also tell you if someone you blocked made a new account, and is trying to pull a fast one. Generally the form and character, and rper style will be almost always match the one that was kicked out. (If your kicked out. Move on, do not harass the admin, other rper's in the rp, or make extra accounts to do it when they mute you. ) If some how the form sneaks by. Most know when they start rping.  Think of roleplay styles like finger prints. Every one has their own. And eventually they slip.

But there are also some forms that can seem.. Over complicated. Where people won't make a form, and join a roleplay cause the form is too long. And some of the details asked will never come up. And some can be deceptive. Like weight. Some people weigh a lot more than they look.  And vice versa.  You could really skip it. If the detail isn't important to the rp you can skip it on the form. 

https://www.youtube.com/watch?v=FF1_IHliRhI