There are many types of data that Google Analytics provides which is extremely useful but here are three types that not checking regularly could mean the demise of your website:
All sources of traffic (found via “Acquisition” > “All Traffic” > “Source/Medium”)
This shows you which sources provide your website with the most valuable traffic which is ultimately that which results in conversions. With this data you can then spend your precious time on the traffic sources that work best for you. Spending time on something that provides no valuable is worse than doing nothing at all in my eyes. Especially when you have easily available data to tell you what works.
Pages viewed (found via “Behavior” > “Site Content” > “All Pages”)
The information within this section shows you what people are interested in. This means that you can tailor future content and marketing efforts around this and so increase your website visitor’s level of. It’s just a case of being nice and helping people out.
Technology used to access your website (e.g. desktops/tablets/mobile/tvs etc)
People behave very differently when using different types of devices and this should be reflected in the experience provided by your website.
For example, if many people are using a mobile phone to access your website there is a high likelihood that they are on the go so you may want to produce content that can be taken in quickly as opposed to having long reads.
The technology that people use to access your website could change very quickly so it is essential that you check this data regularly.