It’s important to know when consumers consume your brand. Do they use it largely for a morning snack or for sharing with friends ?
However, some marketers over estimate the degree to which their brand is confined to a particular situation, used for a particular purpose. Worse, they market in such a ways as to make it a ‘self-fulfilling prophecy’ hemming the brand into one situation through advertising nothing else.
In the same way that product categories can be too narrowly defined based on product features (e.g. the chocolate vs vanilla ice-cream categories), categories based on consumption situation can lull marketers into a false sense of limited competition. e.g. nonsense like “Kit-Kat doesn’t compete with Snickers because Kit-Kat is for taking a break whereas Snickers is to satisfy a hunger craving”.
The reality is that few brands are exclusively bought for specific consumption situations, and which brands are bought for which situation varies between consumers and over time.
Yes the same person in the same situation can choose different brands on different buying times.
Marketing Science 
Very useful post. It clarifies the concept of usage situation in plain English. I think that if behavior learning applies in the case of Kit-Kat and Snickers then we do have limited competition.