I know it's been suggested before, and one of the options presented to people asking for the ability to add custom nutrients was a clever workaround using custom biometrics, but I'm gonna suggest it again to bring up some possibilities custom nutrients would create for users.
Custom nutrients can be any supplement; creatine, collagen, turmeric, catechins, etc.. This is the most likely use-case for custom nutrients.
Tracking things like Vitamin K as separate K1 and K2 would also be an option for users if they could add custom nutrients. This could also be used by people wanting to track EPA and DHA Omega-3s separately. Likely there are even more examples each with their own unique niche of users.
Here's where it gets fun though, custom nutrients don't even need to be nutrients! If someone wanted to track the nutrient "Cost" and input a decimal then custom recipes using customized ingredients could easily display a cost per serving. As long as tracking is done accurately this could also produce a daily cost, knowing your food costs per day then has all sorts of possibilities with custom biometrics. I think seeing a rolling average cost per day and an average cost per week would be very cool and help users who struggle to budget for a healthy diet.
As a workaround for #3, just find a column you don't really use and track cost there. Personally I don't care to track Ash so I just use that as my Cost column. Only minorly inconvenienced that I can't rename it or change it from grams to dollars. Even if I have my own workaround I still think this would be a very powerful and useful feature for a lot of users.