As a sole trader is a business account really needed?

I'm living in Ireland and I have been charge of my wife's financials for her small business for a couple years now. To file her taxes I have to give an account what expenses she used for her business in specific categories. Last year, she spent a good amount of money on food and we didn't realise food is not a business expense. So, she spent all this money using her business account that doesn't get written off, the account doesn't know about it, and it seems like it doesn't really matter how expenses are incurred.

So then, is there any legal reason to have a business bank account? If you were very diligent in separating and organising only from a personal bank account hypothetically you could just do that right?