Every Input Node can be marked as optional so a user is able to skip it. Simply enable the Optional input toggle under the answer options in your Input Node.
This can be useful if you want to add text fields for additional information or comments that a user may not always want to provide. A File-Upload can be also be marked as optional, so a user can upload documents only if they need to do so. Another use case is the ability to add additional terms in a contract that are optional - in this case it would be recommended to use multiple text fields in a Multi-Input.