Here's an example: You decide spontaneously to travel to the UK from the US after having finishing a grueling contract. Your travel plans are a bit vague because of the short notice. You work on your CV on the flight with a view to your next job.
The border control officer finds your lack of detailed travel plans suspicious. He decides to carry out a search of your luggage and finds your CV. He claims you are coming to the UK to seek work. You protest but you have no evidence to the contrary.
You're refused entry and you will now find it difficult to enter the UK for at least 10 years. You can expect to spend 3 or more hours at the UK border each time you try to enter.
Or, alternatively, none of this happens and instead the officer has a laugh with you about disorganised trips overseas and waves you in.
These sorts of scenarios are very common. People should be aware of the peril of crossing borders and be informed and well organised.