The addition of a genotyped biological parent will allow your Ancestry Composition results to be phased and that phasing to be visible in the Inheritance section of this report. Phasing refers to the process of separating the mixed genotype results for each location into the two haplotypes, one from each parent. You can then see the contribution from each of your parents to your overall Ancestry Composition:
Sharing with biological children will also yield higher-resolution results in Ancestry Composition, but this split view of your Ancestry Composition requires the addition of a biological parent. Your ancestry by each parent will be calculated automatically once you establish sharing with at least one parent, or if your parent is on the same account; if the other parent is added later, the split will be recalculated.