When you look at all the different CRM, practice management, and document management systems, they all store and access data differently. Some are cloud based, others are on-premise. Some expose JSON APIs, while others use any number of database technologies. The applications are different, the APIs are different, and the column names are different, yet all the different systems capture roughly the same kinds of information: contacts, matters/projects, tasks, documents, etc.
At a high level, Universal Migrator works by connecting to a source system, copying data out, and transforming it to a Universal set of SQL tables. This saves you tons of time because you don't have to reverse engineer hundreds of different systems and the resulting backup looks the same regardless of what system the data originated in.
Because all backups are structured the same, if you need to tweak or query data, any script you write will work for every system you migrate from.