This paper goes into detail about the war plans of both the British and the Americans. It's quite readable, and if you're interested, I'd suggest taking a look.
For the question itself: interpreting your question as if the US and UK are friends, why bother planning for a war against a friend?, then you'd be right, most of the time both sides thought it was unlikely to go to war with each other, but they thought about it anyway in times of crises. After all, even if you know for sure that you will not resort to force, can you say the same of your erstwhile ally?
The plans of the US and British navies for an Anglo-American war have
received little attention because most historians accept the premise that such an event was 'unthinkable'; that the likelihood of war between the
United States and the British Empire was so remote as to have become an
absurdity. For planners in the armed services, however, who accept as a
matter of course that states use force whenever necessary to further their
interests, it was natural to assume that any other state would, in certain
circumstances, resort to war. And while it is true that British and US
statesmen did not rate highly the likelihood of war between their two
countries, in moments of crisis their thoughts turned to the possibility. No
less a figure than Winston Churchill, the architect of the Anglo-American
'special relationship' and, after 1963, an honorary American citizen, could
write in 1927 that although it was 'quite right in the interests of peace to go
on talking about war with the United States being "unthinkable", everyone
knows that this is not true.'
A second reason is that, no matter how unlikely, if there were a war, the consequences would be dramatic. Think about why people buy insurance for example - sure it's unlikely that your house burns down, but if it does, can you afford to be uninsured?
The consequences of the outbreak of war were potentially so great that
no responsible decision-maker could afford to dismiss the likelihood out of
hand. Americans feared that war with the British Empire might lead to the
destruction of the US fleet and the temporary interruption of US overseas
trade; to attacks on major industrial centres by British air forces based in
Canada; and to the invasion of the continental United States by an imperial
army drawn from Britain, Canada, and the other British colonies. For
Britain, the results would be just as damaging and much more probable:
the destruction of the British fleet, the conquest of British colonies and
former colonies in the western hemisphere, the disruption of Britain's
overseas trade, and, in the event of defeat, an immense blow to British
prestige. Although each of the two assumed that it would never start the
war, each also knew that it could not predict the other's conduct with
Finally, war planners keep "in shape" by planning for war. They are more familiar with what assets they have, what (natural) obstacles they might have to deal with, how long it takes to deploy ships from the Pacific to the Atlantic, and so on:
Indeed, it has been suggested that the 'essential value' of the US naval war plan against Britain - Plan Red 3 - 'was to accustom service planners to dealing with the complexities of an Atlantic centered conflict'
You'll find more examples later in the document. For example, British war planners thought about the threat of the US moving a fleet from the Atlantic to the Pacific, via the Panama Canal. This is not just dangerous to British trade, it might have led to the capture of some colonies such as Hong Kong. As a result, British planners thought about capturing the Philippines to deprive the American navy of bases. Either way such a war is going to be complex, and of course, when it's complex, it's necessary to think about it.