Having a high stockpile allows you to start construction immediately instead of the Trade AI slowly buying the resources on the market.
I use it especially for naval bases, I stockpile all the materials while researching the techs so the bases start building immediately, even preventing shortages to delay construction. Then during the time between techs level the stockpile slowly rises again and is ready for the next round of upgrading.
You can also stockpile steamer convoys because they're usually in short supply and if you want to colonize Africa you'll have to build a lot of ships in the 1860s. Having a high stockpile prevents the situation when a crisis war erupts and suddenly there's no steamer convoys available during 3 years, these 3 years happening to be the ones when you'd have needed to build your colonization fleet
I use it especially for naval bases, I stockpile all the materials while researching the techs so the bases start building immediately, even preventing shortages to delay construction. Then during the time between techs level the stockpile slowly rises again and is ready for the next round of upgrading.
You can also stockpile steamer convoys because they're usually in short supply and if you want to colonize Africa you'll have to build a lot of ships in the 1860s. Having a high stockpile prevents the situation when a crisis war erupts and suddenly there's no steamer convoys available during 3 years, these 3 years happening to be the ones when you'd have needed to build your colonization fleet