Search history, ad clicks, etc. can serialize to very tiny records. I don't think this is about saving storage space. One day of YouTube uploads probably takes more space than every user's data combined.
Sure. Still saves space. Also closes over the envelope of their legal exposure.
Having exabytes of storage space and saving a few TB does functionally nothing.
It'd be like you deleting a single doc file.
Your comment made me wonder how much cost this may save, in terms of storage, maintenance, electricity, etc. Are we talking petabytes of storage? Could the data include things like imagery or massive encapsulations?
There is no guarantee that this feature doesn't just flip a "deleted" bool column in their database outside of GDPR countries. And maybe derived products of the data stick around for longer, i.e. weights of custom ML models, HMM states, etc.
It's also buying good will with users and regulators since privacy became a hot topic in recent times. Google decided to make it a default to delete any data older than 3 or 18 months. This is a clear sign that even 3 month old data has just marginal value, if any.