What exact purpose has the sync option "Background data"?
From the developer page, background data is used for "an application that uses the network in the background." For example, a quick search tells me the market requires this to be turned on. Keeping background data set to on is what enables you to keep data from GMail, Weather, Facebook, Twitter, etc. up to date without having to manually refresh/sync each of them. Also realize that battery usage increases with each app or account that is utilizing background data. I notice Twitter has several time increments to choose from in terms of synchronization rate - this helps balance between having the latest data and preserving battery. Hope it helps.