Basicly you want to keep the lines and the font cold, because when beer hits warm line or tap etc it lets out CO2 eg foamy beer, flat beer and huge head's and warm beer (all are bad when getting oneself a beer)
So you want to keep the lines cold, stops the beer warming up
secondly if you can chill the font and taps the beer will be cold all the way to the tap!
keeping the tap cold is nice but you can get away with out it, as the beer cools it quick, esp if you are pouring often enough.
So best is cold lines beer and tap
2nd best is cold lines and beer
3rd which most deal with (inc. myself so far) is cold beer
the longer the lines of course the more the beer can warm up YUCK esp. in summer
So there you go, thats they reasons why!
How? you pump cold water around the lines to keep them and the beer in them cold!, you can continue to have the cold water travel to chill the font, then the slightly warmed water by this stage returns to the fridge "bucket" to chill again
so cold water from the fridge transfers its chill to the lines and taps, then returns warm to the fridge to chill again
(you need to caluclate the amount of water V the amount you pump V the temp and insulation of lines etc)
bit of trial and error
P.S Linz thanks for the tip i might end up doing this re. the fig eight and the fridge seal