Old Computers are usually rather big, heavy and a bit limited in their ability in terms of interfaces. Like they often have no sd-card-slot, bluetooth or wifi. And yes, they need more power.
This RP400 is small, can be taken everywhere by kids, has all necessary interfaces to make something funny and can on worst even run from a manual energy-source (which of course is more relevant for a poorer country than a rich country).
I can see kids taking this RP400 around like a (paper)notebook, work with them at school, home, their friends home or somewhere else and motivating them to use them more than some stationary device. And because of the hdmi they even can switch places fast if neccessary, limiting the interuptions in their work.