Pros and cons for an extra phone number (dual Sim)

Every time I buy something online, or make an account for an example month bus ticket, they "need" my phone number. I always use alias emails, but I don't have an alias phone number. I know, there are some online phone nr service, but they mostly dosnt work, outside of the US. So I was thinking about getting me a second nr, just for thoese cases were I have to log in. I would by the nr, in cash, and there is nothing data they have to make the nr. But what are your thoughts? What do you see as pros and cons for getting a second nr.? Does it even make sense, when the simcard is in the same phone?