Not all your contacts will want to give you all the information you may be requesting. This means when you go to send a personalised email, if they don’t have an entry in the field you are including in the email, it will have a blank space.
To get round this, it is important that you set a default value for all your custom fields. What default values do is when a contact does not have a value for the field; it will insert the default value instead.
For example, if you had an email where the opening line is “hello %%first name%%” if you don’t set up a default value, it will read “hello”. If you were to set a default value like “sir/madam”, when your contact has no value it will default to “hello sir/madam”