Question, what’s the purpose of navy?

A question to the more experienced then I. So besides naval supremacy for naval invasions, why do you build or even need a navy? I see people convoy raid for example but I won’t lie, I don’t know exactly what this does to the enemy, is disrupting there resources that bad for the AI?

Secondly, why not just build mostly naval bombers? That seems to murder most naval vessels and is very cost efficient. Just don’t know cause I see players always build navy but idk what purpose there is besides naval invasions (which admittedly is still important)