If you start segmenting your target audience when you are working on the marketing activities you are already late.
Different conversion of different segments may require the construction of fundamentally different sales funnels. Having one funnel with different messages may not be sufficient. Or similar funnels could have radically different costs at the same stages and different conversion values between the steps.
All the above means that for different segments, you can end up with different customer acquisition costs. You can discover that some segments will be unprofitable. The remaining ones could be too small to build a business around, due to their small size or low average check.
Therefore, you should segment your customer base, plan sales funnels, work on pricing, and conversions at the stage of unit analysis of a business model before launching your product.
This rule may seem obvious, but I still see it violated all too often in real life.