The first commit to NodeBB was nearly four years ago, and in that time, many changes have been made to the core code itself, from feature additions and bug fixes, to bundling of must-have plugins for all installations.
As with any code that matures, schema changes needed to be made over time to ensure that stored data was kept in an ideal manner, so as to reduce the use of anti-patterns such as god tables and XYZ. The second reason schema changes are made are due to revisions in the original implementation. Perhaps a design decision from before could have been done in a more efficient way, and that may need a migration of active data from one data type to another (e.g. a list to a sorted set).
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