whats that pipe that is branching off my bath water feed?

[tonyboy]

i'm rummaging around under the bath wanting to change the old 1970's taps. the feed to the taps is 15mm but there's another 15mm pipe branching off and into the wall about 6 inches before the feed reaches the tap connector. i have no idea why or what it is for. why would you have a pipe taking water away from the flow to the bath?
i'm not a plumber and cant understand this idea at all. the house used to be two flats and the pipe re-appears in a room next to the bathroom. i'm told this room used to be a kitchen. does this explain anything? do i have to plumb in this "branch" pipe when i connect the feed to the bath taps? or can i cap it off and forget about it?

please help as it is really holding up progress

 

[rpm]

Is the pipe that reappears in what used to be the kitchen blanked off or connected to anything, would you want a water feed in this room for anything?

 

[king of pipes]

Impossible to say but it could be the supply from the lower floor to above often a supply is needed to feed water storage tanks in the loft . Kop

 

[JCplumb]

If you're competent enough, just see what happens.
Plumb your new taps in and see if everything still works, if it doesn't then reconnect the branch. (Put a stop end on the branch when testing, it could be live.