The short answer
An Article 4 direction removes certain permitted development rights, meaning planning permission may be required for changes normally allowed.
Without an Article 4 direction, many small changes to a property don't need planning permission at all. Where one is in place, the same changes suddenly do, which can catch out anyone assuming standard permitted-development rights apply.
Why this matters for your purchase
An Article 4 direction is exactly the kind of designation PropM8's planning constraints section checks for, since it changes what work needs permission on a property you might assume is unrestricted.