Not only does it give the assignee peace of mind but in most circumstances, it is required by law.
For example:
Trade marks- Before IP Australia will accept a ‘Request For a Full Assignment or Transmission of a Trade Mark’ (the form required to give effect to the transfer), they require written and signed deed or agreement.
Copyright – The ownership of copyright is governed by the Copyright Act 1968 (Cth) (Act). Simply paying for the development or production of some form of literary work, computer code, photograph, drawing or design does not entitle you to ownership. Copyright must be assigned in writing and satisfy the formal requirements of the Act. This can be achieved in a Services Agreement or by an Intellectual Property Assignment Deed.
If you are hiring freelancers, purchasing a business or having software developed for you by a third party, you will likely need to consider a formal IP assignment deed or IP assignment agreement. It is also common to assign the IP to a separate entity for the purposes of Intellectual Property Protection.