Hot and Cold Supply to hot tap

[MPlace]

I have a strange set up at my kitchen sink. The hot tap is supplied from both the hot water tank supply, but has a cold supply tee'd in almost like a mixing function. Cold supply is a single mains cold water supply.
1. Any reason why this is the case? Is this a common practice?
2. Are there any implications with me isolating the cold supply?

The set up is fine in the summer when the cold supply is warmer. During the winter it really affects the hot water temperature through the tap. I am considering a mixing valve.

 

[cross thread]

It wants isolating.

 

[Madrab]

Absolutely no reason to have that, especially on a gravity system.

1) No, it isn't common practice.
2) Only implication would be to obtain hotter hot water.