The Linchpin User Profile installation won't destroy the old Confluence user profiles structure.
What is the help text?
The help text is an info with instructions on how to fill out the profile. It is placed inside the edit mode of the user's profile.
It's advisable to use this function and help your users. You could, for example, explain why some profile fields can not be edited (if that's the case for your setup) and who shall be contacted if anyone needs help.
You can use HTML markup inside the help text.
How to activate the help text
Navigate to Confluence administration → Linchpin User Profiles → Profile assistance.
There you will find a text field with the headline Help text "edit profile".
Write your help text into this text area. If you don't write anything, the default text will be displayed (see screenshot above).