Why it's important
Site performance won't remain static. User expectations and behaviors change constantly due to market trends, pop culture and technology disruptions.
Are the optimizations you've made still impacting your goals? How has the shift to mobile changed user engagement and site conversion rate? Are your target keywords still relevant to your audience?
Getting it done
Compare performance on a wider matrix than just daily or weekly. Compare your KPIs week to week, month to month and year to year. This will allow you to see any short and long-term trends, and whether or not changes in performance were isolated blips or signs of changing trends.
Chart your metrics together so you can compare how each one impacts the others - a new keyword could bring in tons of new traffic that doesn't convert, but this won't help you achieve your conversion rate goal.
Research industry benchmarks and conduct competitor analysis to learn where you stand compared to your competition, where you need to improve and to find gaps in the market that you can exploit to outmaneuver your rivals.