Google without Gmail
It might seem crazy but you can have Google services without Gmail.
Gmail is the Google Email service that scans your incoming mail so you can receive relevant adverts. You can find stuff with Google search by "Googling" but to pay for the "free service" you are the product. You are sold to advertisers, your searches are monetised, the results are prioritised to fit Google's business needs.
That's the deal. Google services paid for by advertising. Some people do have a moral stance on this and don't use Google but others want to just use services that are genuinely useful.
The problem with signing up with Google is you will often get an unwanted and unnecessary email address. Windowsphone users that have moved to Android probably only need the Google Play Store to download apps. Most everything else they use is in Microsoft apps - including email. Although, to be fair to Google, the Maps and Photos apps are pretty great!
What people don't realise is you can actually sign up for Google services with other email addresses such as Outlook.com, Hotmail or Yahoo.
Just go to sign up for Google without Gmail and use any email address you have. You can even use a work email address if the only thing you want to do is sync your browser bookmarks in Google Chrome.
Another legitimate reason for having an extra Google account might be a teacher wanting a YouTube channel for showing videos created for students that are separate from their regular account. There could be many others.
If you have a reason to tie Google services to another account. This could be the way to do it.