Taking vitamin B complex as a routine can be a good thing, especially in patients with gastro problems and all chronic diseases and for sure for parents on multiple medications.
A therapeutic trial with thiamine should always be done in case of doubt, as a delay in administering thiamine can lead to severe disease or even death.
I think that ANY patient with acute [severe] heart failure should be given thiamine by injection directly before waiting for lab results.
This may sometimes be life-saving when all other treatments fail to help.
Pregnant women with severe vomiting should also receive thiamine as a delay in this can, in rare cases, cause severe brain damage: Wernicke’s encephalopathy.

Mild thiamine deficiency is quite common, especially in elderly individuals and people who indulge in alcohol or with poor nutritional habits.

In short: administering thiamine may sometimes be an EMERGENCY and may save a life.