YSK: You don't need Gmail to create a Google account
Why YSK: A Google account is not the same as a Gmail account, and you don't need to create a new Gmail email account to have a Google account.
I've spoken to many different people who equate a Google account to a Gmail account. This is not the case. Unfortunately, this leads to many new Gmail accounts being created, solely because they think that they need to have a Gmail in order to access other Google services.
Here's the gist:
A Google account gives you access to Google services like Sheets, Docs, Calendar, Drive, Photos, Gmail, YouTube, etc.
A Gmail account is an email service account with Google. When you create a Gmail, you also create a Google account by default.
However, the reverse is not true. If you create a Google account (with a non-Gmail email address), it does not automatically give you a Gmail account.
If you already have an email account with another provider, you can use that as your Google account.
You're making a new account either way. Whether you login with a gmail or other email address makes no difference to it being a Google account. Making a gmail you ALSO get another email, which you can promptly ignore until some possible point in the future where it may be useful.
I mean if you make a gmail account just for your Google account, you'll either have to log into your Gmail account to check emails or miss potential important emails related to your account, which you would get if they were sent to your main email.
I suppose "harm" can be subjective in this context, and there are already some good replies here.
One more thing to add to the list that I'd consider harmful in creating a Gmail account is all of the privacy issues that come with having a Gmail account.
Out of respect for my recipients and myself, I wouldn't want all of our emails being read.
We can go down the rabbit hole of "Email is inherently insecure anyways," but that's a separate discussion.