All combustion type furnaces require combustion air, depending on age of house, local codes etc. Sometimes surrounding air is adequate, sometimes you require "make-up" air, which is why you have that kit available. The experts here will tell you more, presumably, if your boiler was installed professionally, you have what you need.

The main thing is to ensure that there's enough air entering through leakage or intended vents, so that products of combustion - CO2, CO etc are not being sucked back into the house from the furnace/boiler etc.

In newer houses which are quite airtight in comparison to older ones (or should be) this is very important for safety. In some you will see a metal duct that leads from the outside to the furnace room, with a cage over the end, this provides combustion air.

Did you know there is also an insulating cover for your Beckett if noise is an issue?