The main reason anti-choice campaigners want to declare that the concept of a human being begins at conception (or in the case of some at ejaculation) is to claim foetal rights. Their claim always, without exception gives the zygote, blastocyte, and all following stages complete rights and the soon to be pregnant or pregnant women no right at all.
Stephanie Zvan at Almost Diamonds has a couple of graphs that demonstrate this attitude.
This graph looks at the human rights of the two entities as seen by the anti-choice campaigners, based on the concept of no abortions following foetal viability. I would argue that for those with this attitude, the lines would be closer to vertical.
This one looks at the same situation while maintaining autonomy for the women.
In Stephanie’s words:
It is the argument that once the fetus has rights, the pregnant person no longer does. They no longer have any say in how their body is used. They no longer have any right to say that pregnancy is not in their interests–medical, emotional, social, or financial. They have no rights at all in the matter.
That’s what a law banning third-trimester abortions does. It assigns 100% of the rights to the fetus, not 50%.
Those who argue that life begins at contraception or earlier would have the crossover point at zero, with the lines exactly vertical. They see no gradient, no grey areas, only the complete and absolute transfer of rights away from the women.
Here’s a graph I made (not nearly as smooth as Stephanie’s)
Again the lines should be vertical, but the point of the transfer of rights is demonstrated very clearly by these charts. Since many people who are against abortion are also against contraception, in their minds the rights of the women regarding reproductive autonomy are exactly zero at all points from birth to death.