"Do less with less" is a good way to put it. It can be helpful to look at the extremes- for example, people who are starving can end up with suppressed thyroid function (among other things). This appears to be adaptive; a hypothyroid state will tend to avoid building or sometimes even maintaining metabolically expensive muscle, overwhelming exhaustion will tend to suppress nonvital calorie expenditure, and even fidgeting behaviors can be suppressed. In other words, by reducing burn rate, you starve slower.

This is not something you want happening in a well-nourished individual. Beyond making you more likely to die to predation or accidents from severe muscle wasting, it also just feels horrible. There's a reason why people with untreated hypothyroidism (unrelated to starvation) struggle with exercise and weight loss.

I've also personally observed some people on... inadvisable extreme crash diets getting some weird bloodwork numbers. Like TSH spiking by a factor of 10- which, unlike the above starvation case which typically suppresses TSH, may imply malnutrition and inability to produce sufficient thyroid hormone. Their empirically derived caloric burn rate dropped by more than 30% over the duration of the diet, and a substantial amount of that was from dramatic muscle wasting. Not exactly ideal!