Per Y'shtola's special eyes, when a person who has become an abomination dies, their soul itself is destroyed. Meteion had a way to destroy souls permanently. Despite having this method, she felt the need to trap souls in order to prevent them from being reborn.

Now these souls were also used to power her song of Oblivion (maybe? implied but don't think ever stated), but I was under the impression she was concerned about souls being reborn. Made her big speech on how even if all life on one planet was destroyed, the souls/aether would eventually be drawn to another world to be reborn. Why was that a concern when she can just delete souls?