General homebrew discussion, tips and help on kit and malt extract brewing, and talk about equipment. Queries on sourcing supplies and equipment should go in The Store.
The main reason is to increase the malt character of your brew to give more body and flavour. You are replacing the kilo of sugar / dex / BE 1 or 2 etc with another can.
It also gives you the combined hop IBU's from the 2 cans, but as Kevnlis said, this is balanced by the increase maltiness.
You could achieve the same result by using 1 kit can, and top it up with unhopped dry or liquid malt extract, and choose your own additional hops to up the IBU level and hop flavour/aroma you need to achieve a balanced brew.
Using 2 cans just makes it easier. No mucking around with dry ingredients etc.
You could also make a brew by using the Morgans or Coopers cans of unhopped malt extract, say 2 cans of light, 1 can of amber, and add lots of hops such as Northern Brewer or Chinook for bittering, and Cascade for flavour and aroma. Ferment with US05, and you'd have a lovely American Pale Ale. Change the hops to say Fuggles, Goldings, and Styrian Goldings with say Nottingham yeast and you'd have an English Bitter etc.
There are lots of possibilities, with very little extra work compared to a simple K&K.