Prior to XML the way a lot of data got transferred was via comma-separated-value files. They have a number of limitations (to put it mildly), like how to store strings (single quotes? double quotes? no quotes at all?), what character to use to separate values (it wasn't always a comma - sometimes semicolons and pipes were used), and how to escape the in-band control characters (commas, quotes if used, newlines). If you had to share a file with another party there was a negotiation that happened ahead of time on how all that was to be done.

Example: You have to send the name of customer Finley O'Conner. Do you send it as:

  ,Finley O'Conner,
  ,"Finley O'Conner",
  ,'Finley O''Conner',
  ,'Finley O\'Conner',

And so on. But with XML the process of escaping control characters was well defined, and the receiver would commonly publish a schema definition that detailed what all the values were like (types, sizes, etc.) You could be up and running fairly quickly and with some confidence that the integration would work.