To fit with the characteristics of the drink. Red wine is a wide glass to allow oxygen to mix which makes it taste better. Champagne is tall and narrow so that the fizz lasts longer. Brandy has a strong and flavourful smell so the Brandy snifter is wide with a narrow top to trap the air and concentrate it at your nose when you smell or sip.

Can anyone explain why this is a/r/askhistorian question and not just /r/answers?