Google Analytics data (and website analytics in general) are one of the best ways of tracking the performance of your site and see what is working with your online audience. Not only can they show what is working, but they can also show what isn’t (aka areas for improvement).
But if you throw referral spam into the mix, the accuracy of this data is at stake.
You could be getting a decent number of sessions that are basically junk visits.
If you don’t have the best data available in Google Analytics, you risk making bad decisions. Basically, because the data isn’t accurate, you can misinterpret things.
For example, you could spend extra time and money on improving a particular page on your site because it looks popular with your audience. But if all of this traffic is coming from bots, there is no point investing in improvements.
In summary: cleaning up your data, spam bot filters ensure your analytics deliver insights that are more accurate and useful.