Sorry for my ignorance - especially after being in this area for a long time.
Why is it useful to store data in unusual projections and datums? I understand the value of projections as output constructs, because of distortion, etc: that much is justifiable.
However, I don't understand why, for instance, states use state plane projections in data. We have decimal accuracy: why not just store precise values in EPSG:4326? Is this entirely a remnant of pre-auto-reprojection days, or is there a value proposition I'm missing?
edit: I'll restrict the scope of this question to vector data only to make it more concrete. Thanks whuber for the tip.