Value in the community lifecycle is tricky to measure. Value and how you define it depends on what stage of the lifecycle your community is in. It is ever-changing, from the first launch to growth to maturity. You need to prove the value of your community if you want it to stick around and more importantly, grow.
The first stage of your community is actually getting it launched. You will undoubtedly get questions about how the site is doing, whether it is growing, and how you can prove that it is succeeding. Luckily there are hundreds of different metrics and data points you can look at when starting up a site.