For Aramaki, it was to display his powers and beliefs. He beat injured King and Queen, was pressuring the Scabbards and was handling things pretty well until Shanks came, plus went on about his ideology regarding absolute rule and oppressive ruling bringing order.
On the protagonists' side, he was here to show an Admiral isn't something Luffy, his crew or the alliance will cower at. Wano residents did their best protecting their nation and whilst they didn't succeed on their own, they put up a fight and Momo showed off a strong attack and the capacity to frustrate Aramaki and destroy his tree-man.
He'll show off more in the future, I guess.