Your ability to sustain an empire depends, to a large extent, on how well you can compete on the high seas. This is the era of truly global trade, and to keep that trade up, you need a strong Navy.
Navies are made up of Flotillas, which are constructed and maintained by Naval Bases. Naturally, these can only be built on coastlines, where they consume military vessels such as Man-o-Wars or Ironclads constructed in Shipyards.
Like Barracks they also employ Servicemen and Officers Pop, and depending on your navy configuration may need to consume other military goods as well, such as Ammunition and Radios, in order to keep in fighting condition. Flotillas differ from Battalions in how long it takes to create and upgrade them; constructing a competitive navy is not something you can begin considering when your rival has already started saber-rattling.
Another difference between Battalions and Flotillas is that your country’s navy is always considered to be in full active service. In peacetime Generals can keep their troops on low alert, limiting their consumption and expenses. Once war breaks out, Generals can be selectively mobilized to only deploy the troops necessary. Admirals on the other hand have exorbitant needs and expenses even while at peace, so sizing and updating your navy's technology appropriately is an important consideration for imperialists on a tight budget.
On the other hand, navies provide you with considerable Power Projection which confers substantial Prestige onto your country. Having a world-class navy is not strictly a requirement to be a Great Power, particularly if you’re a large self-sustaining terrestrial empire, but it definitely helps you both gain and hold onto the title.