When you or your client signs a Joist estimate or invoice, that signature is an acknowledgement of the contents of the document at the time of signing.
That means if the document is changed in any way, the signature would have to be removed so that either party can review the changes and sign again if need be.
Of course there may be times when you want to keep the original document with the signature in tact. This is why when you make a change to a signed document in Joist and save, Joist gives you two options:
Remove the signature and update the document
Duplicate the document and remove the signature from the duplicate
With the second option, you get to keep your original document with the signature, while also getting a duplicated version of the document without the signature, including any changes you made. The best of both worlds.