Why buy to let?

Can I just ask the purpose behind buy to let? I've been told buy to let's are interest only mortgages, so capital is not being repaid.

Is it just an alternative to putting money in savings, or overpaying a residential mortgage?

I know the house price might go up, but it also might go down?

I know there might be a monthly taxable profit, but it's based on a big deposit, and what if interest rates go up and eat into this profit?

So if an expert here can give me an answer that would be good as I have some money to invest but not sure what to do with it yet.