I installed Google Analytics as a WP plugin just to have a nice source of info every time I work on my site without having to log into my web host's control panel. Through my SEO plugin, the required code from Google is injected into every site.

I'm getting severely confused about the accuracy of traffic reporting.

For example Google reported 19 visits yesterday while my web host showed 198 - over 10 times more than Google.

Is normal? If so, why do we or anyone who runs a web site online even worry about SEO if we get data that varies so much?

I'm not an expert at this, but I think that it all depends on how you define a "visit".

It is possible that your server stats include visits by bots.

I got exactly the same situation here, and it's not only because of different definition of  a visit, from Bluehost server i get visits from more countries than google, and they count robots , but separately so you can see the portion that is not from robot, they even count search from google , for example as 60, and on goggle itself it's reported 2. Very strange, if anyone knows anything about this it please share.
i know for sure for one instance that i have a buyer from Serbia, the sale gone through perfectly, the user also registered on my site a few days before the sale, and google did not register any visit from this location, nor any sign- in page or check- out page.