1. Sourcing is the most important activity for a VC investor. I would say that it's far more important than helping out post-investment, if I were to be honest. Here's why.
2. The existence proof of 'adding value' is backing a company that will create that value when others won't. You build the conviction to do that through deep research and lots of digging. As one founder once said to me, "the first investor is the only investor that 'adds value'."
3. Any founder that knows a sufficient amount about their domain will know more than you about that domain so there's just very little 'value' you can add. Put another way, if you know more than a founder about their domain, maybe you should be the founder!
4. Balancing this out: I think that investors do add a lot of value at points where a lot of value can be created or destroyed, i.e. 'inflection points' such as raising money when the company needs it, sourcing/negotiating an acquisition offer, or hiring primary/key team members.
5. These are just some thoughts around investing in very early stage companies. Once a company has found product/market fit, then there is quite a lot that outside parties can do to help it scale. Before that point, it's all about the picking the right founders and market.