It matters, just not in as much as one might hope. It is mostly a symbolic step, true, because no one will arrest a sitting president. But it shows that the court is observing and condemning the actions. Think of what might come after: Trump couldn’t enter quite a few countries without fear of arrest once he is no longer president. Same with Putin, or Netanyahu. It makes international action so much easier if you can just say “well, there is a warrant out for you, so you better not show up”. Become a refugee? Better not go to a country that might uphold the warrant. Regime change in your current country of residence? Better think fast, the new administration might just need a reason to take you in.