Because the UK chose to not be a part of the single market anymore, so goods they UK wish to export into the EU needs to be treated like any other 3rd country goods with checks and certifications to prove that they are good enough, but with NI still part of the single market as well as the UK, this label was the best solution they could come up with to avoid having to check everything going from the UK to NI. Without the label trade friction will be lower between Ireland and NI, which will please republicans, but not unionists I think.
Because the UK chose to not be a part of the single market anymore, so goods they UK wish to export into the EU needs to be treated like any other 3rd country goods with checks and certifications to prove that they are good enough, but with NI still part of the single market as well as the UK, this label was the best solution they could come up with to avoid having to check everything going from the UK to NI. Without the label trade friction will be lower between Ireland and NI, which will please republicans, but not unionists I think.